Add a new method with eval


class String
  eval %{def last(n)
           self[-n, n]
         end}
end
"Here's a string.".last(7)               # => "string."

1.	Use eval to run a statement dynamically
2.	Execute an eval in a thread
3.	Evaluation with variable
4.	eval with variable name

Add a new method with eval : eval « Development « Ruby