This is the essence of Newton's Third Law of Motion:

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

Here's how it works:

* Action Force: When one object exerts a force on another object (the action), the second object simultaneously exerts an equal and opposite force on the first object (the reaction).

* Equal and Opposite: This means the forces have the same magnitude (strength) but act in opposite directions.

Example:

* You push a wall: You exert a force on the wall (action). The wall pushes back on you with an equal and opposite force (reaction).

* A book sitting on a table: The book exerts a downward force due to gravity (action). The table exerts an upward force on the book, preventing it from falling (reaction).

Important Note: The action and reaction forces always act on different objects, never on the same object.