Newton's third law is not about balance: the mistake in almost every first-year answer

Ask a class what Newton's third law says and most will say 'every action has an equal and opposite reaction'. Ask why a book sitting on a table does not move, and most will say 'because the reaction balances the weight'. The second answer is wrong, and the wrong answer comes from the first sentence being incomplete.

The law, complete: when object A pushes on object B, object B pushes on object A with the same size of force in the opposite direction. The two forces of a third-law pair act on different objects. Always. That last sentence is the whole law.

Now the book. The Earth pulls the book down; that is the book's weight. The third-law partner of that force is the book pulling the Earth up, and that force acts on the Earth, not on the book. So it cannot balance anything on the book.

What balances the weight is the table pushing up on the book, the normal force. Its third-law partner is the book pushing down on the table. Two separate pairs, four forces, and the book is still because the two forces acting on the book, weight down and normal up, happen to be equal. That equality comes from Newton's first law, because the book is not accelerating. It is not the third law at all.

How to tell the difference in an exam: a third-law pair is always the same kind of force, between the same two objects, in opposite directions. Gravity and normal force are different kinds, so they are not a pair. The moment you see 'balanced by its reaction', you know the answer is going wrong.

Why it matters beyond the mark: if the third law were about balance, nothing could ever accelerate, because every force would be cancelled. Things accelerate precisely because the pair acts on two different bodies. When you push a wall, the wall pushes you; you slide backwards and the wall does not, because the wall is attached to a building.

The free-body diagram method that makes this automatic is in the Class 11 Physics notes on mechanics I uploaded. Draw one object, draw only the forces on that object, and the third law stops being confusing.