Sir Isaac Newton described it as "when one object exerts a force upon a second object, the second object exerts an equal and opposite force upon the first object." This simply means that more every action there is an equal and
opposite reaction. The resulting force is equal in size but opposite in direction compared to the original force. An example of Newton's Third Law in motiom is the liftoff of the space shuttle. As the shuttle takes off, its engine burns fuel which is released and pushes the shuttle upward. The fuel exhaust has to be released in the opposite direction in order for the shuttle to accelerate and reach the speed needed to leave the Earth's atmosphere and enter outer space.
When realizing that there is a reaction for every action, you must also learn that there are "action and reaction pairs." This term is quite simple to understand and it is even easier to find examples of it in the world around you.
Two examples of ACTION and REACTION PAIRS:
A baseball forces a bat to the right (ACTION)
A bat forces a baseball to the left (REACTION)
A weightlifter pushes a bar upward (ACTION)
A bar pushes downward on a weightlifter (REACTION)