Make it big : Transforming to upper case: Hello World! becomes HELLO WORLD!
Make it small: Transforming to lower case: HELLO WORLD! becomes hello world!
Make rot13 : A Kind of Cesar code, transforming by replacing each character
by the letter 13 places further along in the alphabet(wrapping back if necessary).
Hello World! becomes Uryyb Jbeyq!
And the best: Rot13(Rot13(x)) = x, i.e. Uryyb Jbeyq! becomes Hello World!