Here's why:

* Collisions: Atoms and molecules in a gas or liquid are constantly moving and colliding with each other and the walls of their container.

* Force: Each collision exerts a tiny force.

* All directions: Because the collisions are random, the force is exerted in all directions.

* Pressure: The sum of all these tiny forces acting on a given area is what we call pressure.

Think of a balloon. The air inside is made of gas molecules. These molecules collide with the inside of the balloon, pushing outward. The more collisions, the more force, and the higher the pressure inside the balloon.