As the temperature of a liquid increases, the molecules gain more energy and move faster. This causes the molecules to move around more and collide with each other more frequently. As a result, the liquid becomes less dense and expands in volume.

The faster the molecules move, the more collisions there will be between them. This increased collision frequency causes the liquid to become more viscous. In other words, it becomes thicker and more resistant to flow.

At very high temperatures, the molecules in a liquid may move so fast that they escape from the liquid's surface and turn into a gas. This process is called evaporation.