The Third Law is concerned with how objects push on each other and exchange momentum when they interact.
The third law states that for every force there is an equal and opposite force. For example, if you push on a wall, it will push back on you as hard as you are pushing on it.
Equal and opposite forces
Equal and opposite forces are present when two objects collide, as shown in the following animation.
The above animation shows two objects running into and bouncing off of each other. There would be a small passage of time during which the collision occurs. During this time the balls push on each other. As hard as one ball pushes on the other, the other pushes on the one.
To be completely clear here, understand that the strength of these forces may change during the collision, but these changes would be scaled the same for both forces. If the force provided by the one on the other started out small, built up to a maximum, and finally was reduced, then the push back by the other on the one would start out small, build to a maximum, and reduce.
So, if an apple and a coffee cup ran into each other, during the collision they would push on each other equally and oppositely:
Click here and link to a collection of other ways to say Newton's Third Law. An explanation for each way is included. Hopefully, the explanation will show that each way of saying this law actually describes the same physics.