Answer:
When a doctor hits you with a hammer he is testing this
They can be long or short; they can be grown or bought; they can be painted or left bare; they can be round or square.
It goes up and down the stairs without moving.
What is the center of gravity?
This is a dead giveaway
What never gets any wetter no matter how hard it rains?
Easy
Animal
Difficult
Food
Funny
Kids
Logic
What am I?