Anyone can mash potatoes
You can tuna piano but you cannot piano a tuna.
When you ask them "2+2 is ?":
You can mash potatoes, but you can't pee soup. (sorry sorry. Really. I've loved this joke since I was... oh.. six...)
To get to the other side.
A baked potato.(http://www.spudstravels.com/Travel%20Archive/Caribbean/Jamaicaimages/Cannabis%20-%20close.jpg)
An amputater
You can't mash Frankenstein.
Anyone can mash potatoes!