Newton's third law: forces come in pairs

Push on a wall and the wall pushes back on your hand: that push is what you feel. Two skaters shove each other and both glide away. A rocket throws gas backwards and the gas throws the rocket forwards.

Forces never come alone. They come in pairs, one on each of the two objects that are interacting.

A Newton's third law pair is two forces that:

1. act on different objects (A on B, and B on A);
2. are equal in magnitude;
3. are opposite in direction, along the same line;
4. are the same type of force (both gravitational, both contact, both frictional, both electric...).

They also start and stop at the same instant; neither one comes "first".

Quick trick: write the force as "force of A on B", then swap the letters. "Earth pulls book down" becomes "book pulls Earth up". Done.