Forces always occur in pairs. This is known as Newton's Third Law of Motion, which states:

"For every action, there is an equal and opposite reaction."

This means that when one object exerts a force on another object, the second object exerts an equal and opposite force back on the first object. These forces are called action-reaction pairs.

Here are some examples:

* You pushing a wall: You exert a force on the wall, and the wall exerts an equal and opposite force back on you.

* A rocket launching: The rocket expels hot gas downwards, and the gas exerts an upward force on the rocket, causing it to lift off.

* Walking: Your foot pushes against the ground, and the ground pushes back on your foot, propelling you forward.