diff --git a/Rakefile b/Rakefile
index c96fc5a..d11e41f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,3 +1,3 @@
 task :default do
-  exec "ruby test/fakefs_test.rb"
+  Dir['test/*_test.rb'].each { |file| require file }
 end
diff --git a/lib/fakefs.rb b/lib/fakefs.rb
index 0d85386..f8be311 100644
--- a/lib/fakefs.rb
+++ b/lib/fakefs.rb
@@ -1,447 +1,6 @@
 require 'fileutils'
 require 'pathname'
-
-RealFile = File
-RealFileUtils = FileUtils
-RealDir = Dir
-RealFileUtils::Dir = RealDir
-RealFileUtils::File = RealFile
-
-module FakeFS
-  module FileUtils
-    extend self
-
-    def mkdir_p(path)
-      FileSystem.add(path, MockDir.new)
-    end
-
-    def rm(path)
-      FileSystem.delete(path)
-    end
-    alias_method :rm_rf, :rm
-    alias_method :rm_r, :rm
-
-    def ln_s(target, path)
-      raise Errno::EEXIST, path if FileSystem.find(path)
-      FileSystem.add(path, MockSymlink.new(target))
-    end
-
-    def cp(src, dest)
-      dst_file = FileSystem.find(dest)
-      src_file = FileSystem.find(src)
-
-      if !src_file
-        raise Errno::ENOENT, src
-      end
-
-      if File.directory? src_file
-        raise Errno::EISDIR, src
-      end
-
-      if dst_file and File.directory?(dst_file)
-        FileSystem.add(File.join(dest, src), src_file.entry.clone(dst_file))
-      else
-        FileSystem.delete(dest)
-        FileSystem.add(dest, src_file.entry.clone)
-      end
-    end
-
-    def cp_r(src, dest)
-      # This error sucks, but it conforms to the original Ruby
-      # method.
-      raise "unknown file type: #{src}" unless dir = FileSystem.find(src)
-
-      new_dir = FileSystem.find(dest)
-
-      if new_dir && !File.directory?(dest)
-        raise Errno::EEXIST, dest
-      end
-
-      if !new_dir && !FileSystem.find(dest+'/../')
-        raise Errno::ENOENT, dest
-      end
-
-      # This last bit is a total abuse and should be thought hard
-      # about and cleaned up.
-      if new_dir
-        if src[-2..-1] == '/.'
-          dir.values.each{|f| new_dir[f.name] = f.clone(new_dir) }
-        else
-          new_dir[dir.name] = dir.entry.clone(new_dir)
-        end
-      else
-        FileSystem.add(dest, dir.entry.clone)
-      end
-    end
-
-    def mv(src, dest)
-      if target = FileSystem.find(src)
-        FileSystem.add(dest, target.entry.clone)
-        FileSystem.delete(src)
-      else
-        raise Errno::ENOENT, src
-      end
-    end
-
-    def chown(user, group, list, options={})
-      list = Array(list)
-      list.each do |f|
-        unless File.exists?(f)
-          raise Errno::ENOENT, f
-        end
-      end
-      list
-    end
-
-    def chown_R(user, group, list, options={})
-      chown(user, group, list, options={})
-    end
-
-    def touch(list, options={})
-      Array(list).each do |f|
-        directory = File.dirname(f)
-        # FIXME this explicit check for '.' shouldn't need to happen
-        if File.exists?(directory) || directory == '.'
-          FileSystem.add(f, MockFile.new)
-        else
-          raise Errno::ENOENT, f
-        end
-      end
-    end
-
-    def cd(dir)
-      FileSystem.chdir(dir)
-    end
-    alias_method :chdir, :cd
-  end
-
-  class File
-    PATH_SEPARATOR = '/'
-
-    def self.join(*parts)
-      parts * PATH_SEPARATOR
-    end
-
-    def self.exist?(path)
-      !!FileSystem.find(path)
-    end
-
-    class << self
-      alias_method :exists?, :exist?
-    end
-
-    def self.directory?(path)
-      if path.respond_to? :entry
-        path.entry.is_a? MockDir
-      else
-        result = FileSystem.find(path)
-        result ? result.entry.is_a?(MockDir) : false
-      end
-    end
-
-    def self.symlink?(path)
-      if path.respond_to? :entry
-        path.is_a? MockSymlink
-      else
-        FileSystem.find(path).is_a? MockSymlink
-      end
-    end
-
-    def self.file?(path)
-      if path.respond_to? :entry
-        path.entry.is_a? MockFile
-      else
-        result = FileSystem.find(path)
-        result ? result.entry.is_a?(MockFile) : false
-      end
-    end
-
-    def self.expand_path(*args)
-      RealFile.expand_path(*args)
-    end
-
-    def self.basename(*args)
-      RealFile.basename(*args)
-    end
-
-    def self.dirname(path)
-      RealFile.dirname(path)
-    end
-
-    def self.readlink(path)
-      symlink = FileSystem.find(path)
-      FileSystem.find(symlink.target).to_s
-    end
-
-    def self.open(path, mode='r')
-      if block_given?
-        yield new(path, mode)
-      else
-        new(path, mode)
-      end
-    end
-
-    def self.read(path)
-      file = new(path)
-      if file.exists?
-        file.read
-      else
-        raise Errno::ENOENT
-      end
-    end
-
-    def self.readlines(path)
-      read(path).split("\n")
-    end
-
-    attr_reader :path
-    def initialize(path, mode = nil)
-      @path = path
-      @mode = mode
-      @file = FileSystem.find(path)
-      @open = true
-    end
-
-    def close
-      @open = false
-    end
-
-    def read
-      raise IOError.new('closed stream') unless @open
-      @file.content
-    end
-
-    def exists?
-      @file
-    end
-
-    def puts(content)
-      write(content + "\n")
-    end
-
-    def write(content)
-      raise IOError.new('closed stream') unless @open
-
-      if !File.exists?(@path)
-        @file = FileSystem.add(path, MockFile.new)
-      end
-
-      @file.content += content
-    end
-    alias_method :print, :write
-    alias_method :<<, :write
-
-    def flush; self; end
-  end
-
-  class Dir
-    def self.glob(pattern)
-      if pattern[-1,1] == '*'
-        blk = proc { |entry| entry.to_s }
-      else
-        blk = proc { |entry| entry[1].parent.to_s }
-      end
-      (FileSystem.find(pattern) || []).map(&blk).uniq.sort
-    end
-
-    def self.[](pattern)
-      glob(pattern)
-    end
-
-    def self.chdir(dir, &blk)
-      FileSystem.chdir(dir, &blk)
-    end
-  end
-
-  module FileSystem
-    extend self
-
-    def dir_levels
-      @dir_levels ||= []
-    end
-
-    def fs
-      @fs ||= MockDir.new('.')
-    end
-
-    def clear
-      @dir_levels = nil
-      @fs = nil
-    end
-
-    def files
-      fs.values
-    end
-
-    def find(path)
-      parts = path_parts(normalize_path(path))
-
-      target = parts[0...-1].inject(fs) do |dir, part|
-        dir[part] || {}
-      end
-
-      case parts.last
-      when '*'
-        target.values
-      else
-        target[parts.last]
-      end
-    end
-
-    def add(path, object=MockDir.new)
-      parts = path_parts(normalize_path(path))
-
-      d = parts[0...-1].inject(fs) do |dir, part|
-        dir[part] ||= MockDir.new(part, dir)
-      end
-
-      object.name = parts.last
-      object.parent = d
-      d[parts.last] ||= object
-    end
-
-    # copies directories and files from the real filesystem
-    # into our fake one
-    def clone(path)
-      path    = File.expand_path(path)
-      pattern = File.join(path, '**', '*')
-      files   = RealFile.file?(path) ? [path] : [path] + RealDir.glob(pattern, RealFile::FNM_DOTMATCH)
-
-      files.each do |f|
-        if RealFile.file?(f)
-          FileUtils.mkdir_p(File.dirname(f))
-          File.open(f, 'w') do |g|
-            g.print RealFile.open(f){|h| h.read }
-          end
-        elsif RealFile.directory?(f)
-          FileUtils.mkdir_p(f)
-        elsif RealFile.symlink?(f)
-          FileUtils.ln_s()
-        end
-      end
-    end
-
-    def delete(path)
-      if dir = FileSystem.find(path)
-        dir.parent.delete(dir.name)
-      end
-    end
-
-    def chdir(dir, &blk)
-      new_dir = find(dir)
-      dir_levels.push dir if blk
-
-      raise Errno::ENOENT, dir unless new_dir
-
-      dir_levels.push dir if !blk
-      blk.call if blk
-    ensure
-      dir_levels.pop if blk
-    end
-
-    def path_parts(path)
-      path.split(File::PATH_SEPARATOR).reject { |part| part.empty? }
-    end
-
-    def normalize_path(path)
-      if Pathname.new(path).absolute?
-        File.expand_path(path)
-      else
-        parts = dir_levels + [path]
-        File.expand_path(File.join(*parts))
-      end
-    end
-
-    def current_dir
-      find(normalize_path('.'))
-    end
-  end
-
-  class MockFile
-    attr_accessor :name, :parent, :content
-
-    def initialize(name = nil, parent = nil)
-      @name = name
-      @parent = parent
-      @content = ''
-    end
-
-    def clone(parent = nil)
-      clone = super()
-      clone.parent = parent if parent
-      clone
-    end
-
-    def entry
-      self
-    end
-
-    def inspect
-      "(MockFile name:#{name.inspect} parent:#{parent.to_s.inspect} size:#{content.size})"
-    end
-
-    def to_s
-      File.join(parent.to_s, name)
-    end
-  end
-
-  class MockDir < Hash
-    attr_accessor :name, :parent
-
-    def initialize(name = nil, parent = nil)
-      @name = name
-      @parent = parent
-    end
-
-    def entry
-      self
-    end
-
-    def inspect
-      "(MockDir name:#{name.inspect} parent:#{parent.to_s.inspect} size:#{size})"
-    end
-
-    def clone(parent = nil)
-      clone = Marshal.load(Marshal.dump(self))
-      clone.each do |key, value|
-        value.parent = clone
-      end
-      clone.parent = parent if parent
-      clone
-    end
-
-    def to_s
-      if parent && parent.to_s != '.'
-        File.join(parent.to_s, name)
-      elsif parent && parent.to_s == '.'
-        "#{File::PATH_SEPARATOR}#{name}"
-      else
-        name
-      end
-    end
-  end
-
-  class MockSymlink
-    attr_accessor :name, :target
-    alias_method  :to_s, :name
-
-    def initialize(target)
-      @target = target
-    end
-
-    def inspect
-      "symlink(#{target.split('/').last})"
-    end
-
-    def entry
-      FileSystem.find(target)
-    end
-
-    def method_missing(*args, &block)
-      entry.send(*args, &block)
-    end
-  end
-end
+require 'fakefs/safe'
 
 Object.class_eval do
   remove_const(:Dir)
diff --git a/lib/fakefs/safe.rb b/lib/fakefs/safe.rb
new file mode 100644
index 0000000..3df0e50
--- /dev/null
+++ b/lib/fakefs/safe.rb
@@ -0,0 +1,470 @@
+RealFile = File
+RealFileUtils = FileUtils
+RealDir = Dir
+RealFileUtils::Dir = RealDir
+RealFileUtils::File = RealFile
+
+module FakeFS
+  def self.activate!
+    Object.class_eval do
+      remove_const(:Dir)
+      remove_const(:File)
+      remove_const(:FileUtils)
+      const_set(:Dir, FakeFS::Dir)
+      const_set(:File, FakeFS::File)
+      const_set(:FileUtils, FakeFS::FileUtils)
+    end
+  end
+
+  def self.deactivate!
+    Object.class_eval do
+      remove_const(:Dir)
+      remove_const(:File)
+      remove_const(:FileUtils)
+      const_set(:Dir, RealDir)
+      const_set(:File, RealFile)
+      const_set(:FileUtils, RealFileUtils)
+    end
+  end
+
+  module FileUtils
+    extend self
+
+    def mkdir_p(path)
+      FileSystem.add(path, MockDir.new)
+    end
+
+    def rm(path)
+      FileSystem.delete(path)
+    end
+    alias_method :rm_rf, :rm
+    alias_method :rm_r, :rm
+
+    def ln_s(target, path)
+      raise Errno::EEXIST, path if FileSystem.find(path)
+      FileSystem.add(path, MockSymlink.new(target))
+    end
+
+    def cp(src, dest)
+      dst_file = FileSystem.find(dest)
+      src_file = FileSystem.find(src)
+
+      if !src_file
+        raise Errno::ENOENT, src
+      end
+
+      if File.directory? src_file
+        raise Errno::EISDIR, src
+      end
+
+      if dst_file and File.directory?(dst_file)
+        FileSystem.add(File.join(dest, src), src_file.entry.clone(dst_file))
+      else
+        FileSystem.delete(dest)
+        FileSystem.add(dest, src_file.entry.clone)
+      end
+    end
+
+    def cp_r(src, dest)
+      # This error sucks, but it conforms to the original Ruby
+      # method.
+      raise "unknown file type: #{src}" unless dir = FileSystem.find(src)
+
+      new_dir = FileSystem.find(dest)
+
+      if new_dir && !File.directory?(dest)
+        raise Errno::EEXIST, dest
+      end
+
+      if !new_dir && !FileSystem.find(dest+'/../')
+        raise Errno::ENOENT, dest
+      end
+
+      # This last bit is a total abuse and should be thought hard
+      # about and cleaned up.
+      if new_dir
+        if src[-2..-1] == '/.'
+          dir.values.each{|f| new_dir[f.name] = f.clone(new_dir) }
+        else
+          new_dir[dir.name] = dir.entry.clone(new_dir)
+        end
+      else
+        FileSystem.add(dest, dir.entry.clone)
+      end
+    end
+
+    def mv(src, dest)
+      if target = FileSystem.find(src)
+        FileSystem.add(dest, target.entry.clone)
+        FileSystem.delete(src)
+      else
+        raise Errno::ENOENT, src
+      end
+    end
+
+    def chown(user, group, list, options={})
+      list = Array(list)
+      list.each do |f|
+        unless File.exists?(f)
+          raise Errno::ENOENT, f
+        end
+      end
+      list
+    end
+
+    def chown_R(user, group, list, options={})
+      chown(user, group, list, options={})
+    end
+
+    def touch(list, options={})
+      Array(list).each do |f|
+        directory = File.dirname(f)
+        # FIXME this explicit check for '.' shouldn't need to happen
+        if File.exists?(directory) || directory == '.'
+          FileSystem.add(f, MockFile.new)
+        else
+          raise Errno::ENOENT, f
+        end
+      end
+    end
+
+    def cd(dir)
+      FileSystem.chdir(dir)
+    end
+    alias_method :chdir, :cd
+  end
+
+  class File
+    PATH_SEPARATOR = '/'
+
+    def self.join(*parts)
+      parts * PATH_SEPARATOR
+    end
+
+    def self.exist?(path)
+      !!FileSystem.find(path)
+    end
+
+    class << self
+      alias_method :exists?, :exist?
+    end
+
+    def self.directory?(path)
+      if path.respond_to? :entry
+        path.entry.is_a? MockDir
+      else
+        result = FileSystem.find(path)
+        result ? result.entry.is_a?(MockDir) : false
+      end
+    end
+
+    def self.symlink?(path)
+      if path.respond_to? :entry
+        path.is_a? MockSymlink
+      else
+        FileSystem.find(path).is_a? MockSymlink
+      end
+    end
+
+    def self.file?(path)
+      if path.respond_to? :entry
+        path.entry.is_a? MockFile
+      else
+        result = FileSystem.find(path)
+        result ? result.entry.is_a?(MockFile) : false
+      end
+    end
+
+    def self.expand_path(*args)
+      RealFile.expand_path(*args)
+    end
+
+    def self.basename(*args)
+      RealFile.basename(*args)
+    end
+
+    def self.dirname(path)
+      RealFile.dirname(path)
+    end
+
+    def self.readlink(path)
+      symlink = FileSystem.find(path)
+      FileSystem.find(symlink.target).to_s
+    end
+
+    def self.open(path, mode='r')
+      if block_given?
+        yield new(path, mode)
+      else
+        new(path, mode)
+      end
+    end
+
+    def self.read(path)
+      file = new(path)
+      if file.exists?
+        file.read
+      else
+        raise Errno::ENOENT
+      end
+    end
+
+    def self.readlines(path)
+      read(path).split("\n")
+    end
+
+    attr_reader :path
+    def initialize(path, mode = nil)
+      @path = path
+      @mode = mode
+      @file = FileSystem.find(path)
+      @open = true
+    end
+
+    def close
+      @open = false
+    end
+
+    def read
+      raise IOError.new('closed stream') unless @open
+      @file.content
+    end
+
+    def exists?
+      @file
+    end
+
+    def puts(content)
+      write(content + "\n")
+    end
+
+    def write(content)
+      raise IOError.new('closed stream') unless @open
+
+      if !File.exists?(@path)
+        @file = FileSystem.add(path, MockFile.new)
+      end
+
+      @file.content += content
+    end
+    alias_method :print, :write
+    alias_method :<<, :write
+
+    def flush; self; end
+  end
+
+  class Dir
+    def self.glob(pattern)
+      if pattern[-1,1] == '*'
+        blk = proc { |entry| entry.to_s }
+      else
+        blk = proc { |entry| entry[1].parent.to_s }
+      end
+      (FileSystem.find(pattern) || []).map(&blk).uniq.sort
+    end
+
+    def self.[](pattern)
+      glob(pattern)
+    end
+
+    def self.chdir(dir, &blk)
+      FileSystem.chdir(dir, &blk)
+    end
+  end
+
+  module FileSystem
+    extend self
+
+    def dir_levels
+      @dir_levels ||= []
+    end
+
+    def fs
+      @fs ||= MockDir.new('.')
+    end
+
+    def clear
+      @dir_levels = nil
+      @fs = nil
+    end
+
+    def files
+      fs.values
+    end
+
+    def find(path)
+      parts = path_parts(normalize_path(path))
+
+      target = parts[0...-1].inject(fs) do |dir, part|
+        dir[part] || {}
+      end
+
+      case parts.last
+      when '*'
+        target.values
+      else
+        target[parts.last]
+      end
+    end
+
+    def add(path, object=MockDir.new)
+      parts = path_parts(normalize_path(path))
+
+      d = parts[0...-1].inject(fs) do |dir, part|
+        dir[part] ||= MockDir.new(part, dir)
+      end
+
+      object.name = parts.last
+      object.parent = d
+      d[parts.last] ||= object
+    end
+
+    # copies directories and files from the real filesystem
+    # into our fake one
+    def clone(path)
+      path    = File.expand_path(path)
+      pattern = File.join(path, '**', '*')
+      files   = RealFile.file?(path) ? [path] : [path] + RealDir.glob(pattern, RealFile::FNM_DOTMATCH)
+
+      files.each do |f|
+        if RealFile.file?(f)
+          FileUtils.mkdir_p(File.dirname(f))
+          File.open(f, 'w') do |g|
+            g.print RealFile.open(f){|h| h.read }
+          end
+        elsif RealFile.directory?(f)
+          FileUtils.mkdir_p(f)
+        elsif RealFile.symlink?(f)
+          FileUtils.ln_s()
+        end
+      end
+    end
+
+    def delete(path)
+      if dir = FileSystem.find(path)
+        dir.parent.delete(dir.name)
+      end
+    end
+
+    def chdir(dir, &blk)
+      new_dir = find(dir)
+      dir_levels.push dir if blk
+
+      raise Errno::ENOENT, dir unless new_dir
+
+      dir_levels.push dir if !blk
+      blk.call if blk
+    ensure
+      dir_levels.pop if blk
+    end
+
+    def path_parts(path)
+      path.split(File::PATH_SEPARATOR).reject { |part| part.empty? }
+    end
+
+    def normalize_path(path)
+      if Pathname.new(path).absolute?
+        File.expand_path(path)
+      else
+        parts = dir_levels + [path]
+        File.expand_path(File.join(*parts))
+      end
+    end
+
+    def current_dir
+      find(normalize_path('.'))
+    end
+  end
+
+  class MockFile
+    attr_accessor :name, :parent, :content
+
+    def initialize(name = nil, parent = nil)
+      @name = name
+      @parent = parent
+      @content = ''
+    end
+
+    def clone(parent = nil)
+      clone = super()
+      clone.parent = parent if parent
+      clone
+    end
+
+    def entry
+      self
+    end
+
+    def inspect
+      "(MockFile name:#{name.inspect} parent:#{parent.to_s.inspect} size:#{content.size})"
+    end
+
+    def to_s
+      File.join(parent.to_s, name)
+    end
+  end
+
+  class MockDir < Hash
+    attr_accessor :name, :parent
+
+    def initialize(name = nil, parent = nil)
+      @name = name
+      @parent = parent
+    end
+
+    def entry
+      self
+    end
+
+    def inspect
+      "(MockDir name:#{name.inspect} parent:#{parent.to_s.inspect} size:#{size})"
+    end
+
+    def clone(parent = nil)
+      clone = Marshal.load(Marshal.dump(self))
+      clone.each do |key, value|
+        value.parent = clone
+      end
+      clone.parent = parent if parent
+      clone
+    end
+
+    def to_s
+      if parent && parent.to_s != '.'
+        File.join(parent.to_s, name)
+      elsif parent && parent.to_s == '.'
+        "#{File::PATH_SEPARATOR}#{name}"
+      else
+        name
+      end
+    end
+  end
+
+  class MockSymlink
+    attr_accessor :name, :target
+    alias_method  :to_s, :name
+
+    def initialize(target)
+      @target = target
+    end
+
+    def inspect
+      "symlink(#{target.split('/').last})"
+    end
+
+    def entry
+      FileSystem.find(target)
+    end
+
+    def method_missing(*args, &block)
+      entry.send(*args, &block)
+    end
+  end
+end
+
+def FakeFS
+  return FakeFS unless block_given?
+  ::FakeFS.activate!
+  yield
+  ::FakeFS.deactivate!
+end
diff --git a/test/safe_test.rb b/test/safe_test.rb
new file mode 100644
index 0000000..ac0f63f
--- /dev/null
+++ b/test/safe_test.rb
@@ -0,0 +1,20 @@
+$LOAD_PATH.unshift File.join(File.dirname(__FILE__), '..', 'lib')
+require 'fakefs/safe'
+require 'test/unit'
+
+class FakeFSSafeTest < Test::Unit::TestCase
+  def setup
+    FakeFS.deactivate!
+  end
+
+  def test_FakeFS_method_does_not_intrude_on_global_namespace
+    path = '/path/to/file.txt'
+
+    FakeFS do
+      File.open(path, 'w') { |f| f.write "Yatta!" }
+      assert File.exists?(path)
+    end
+
+    assert ! File.exists?(path)
+  end
+end