Newton's Third Law
"For every action, there is an equal and opposite reaction." Isaac Newton.
This means that for every force there is a reaction force that is equal in size, but opposite in direction. Whenever an object pushes another object, it gets pushed back in the opposite direction equally.
For example:
Homer's leg (action) is pushing down on the Wii Fit. The normal force (see below) is the reaction of the Wii Fit as it pushes Homer upwards with an equal force, which allows Homer to balance himself on top of the Wii Fit.
