Why does a liquid have a definite volume and a shape that can vary?

A liquid has a definite volume and a shape that can vary because the particles in a liquid are kept together very loosely. This allows the particles to move freely around each other. Liquids have a low viscosity, meaning a low resistance to flow.

Let's say you have 20 people and you put those 20 people in a room for an hour. Over the course of an hour, the people would most likely move around the room. But, you would always have 20 people there in the room. The number of people doesn't change unless you add or take away people.

The same goes for a liquid. If you put 20 mL of water in a cup and left it there for an hour, there would always be 20 mL of water unless you added or took away water.

The people would be confined to the room, and the particles of the water would be confined in the cup. If you took the people and put them in a larger room, they would spread out and adjust according to the room because they are free to move. If you took the water and put in in a larger cup, it would adjust according to the space because they are free to move.