require 'stringio'

sio=StringIO.new('acts like a file')
puts sio.pos # => 0
sio.pos= sio.length - 4
puts sio.read # => 'file'

http://whytheluckystiff.net/articles/rubyOneEightOh.html

This is handy for testing, when you need to simulate an open file or open URI.  Why, and the Pickaxe book, have better examples

make Ruby strings behave like files

url, file, ruby

Ruby