Here's why:

* Normal Force: The normal force is a contact force that acts perpendicular to the surface of contact. It's the force that prevents an object from falling through a surface.

* Weight: The weight of an object is the force of gravity acting on it. It always acts downward.

When an object is placed on a flat surface, the surface pushes back on the object with a force equal in magnitude but opposite in direction to the object's weight. This force is the normal force.

In simpler terms:

Imagine you put a book on a table. The book's weight pulls it down. The table pushes back upward with an equal force to keep the book from falling through. This upward force is the normal force.