NoMethodError: undefined method `copy' for File:Class

I wanted to copy a file from Ruby. Documentation for File#copy said that

File.copy(from, to)

should work. But, I got this error message:

NoMethodError: undefined method `copy' for File:Class

Google search led me to ruby-talk post that said I should do:

require "ftools"

I am just surprised that documentation did not make that clear.