The tiny atoms and molecules from which every substance is made are constantly moving.
The speed at which molecules move depends on temperature.
Heat gives atoms and molecules extra energy, making them move faster.
In 1827 Scottish botanist Robert Brown saw through a microscope that pollen grains in water were constantly dancing. They are buffeted by moving molecules that are too small to be seen. The effect is called Brownian motion.
In a gas, the atoms and molecules are so far apart that they are able to zoom about freely in all directions.
As liquids boil, the atoms and molecules move around more and more energetically until some break away altogether and turn to gas. This is called evaporation.
Smells spread quickly because the smell molecules move about very quickly.
In a liquid, molecules are closely packed and move like dancers in a nightclub. If molecules stopped moving in liquids we would all die, because this movement is what moves materials in and out of human cells.
In a solid, atoms and molecules are bound together and vibrate on the spot.
Air and water pressure is simply bombardment by billions of moving molecules.
At —273.15°C, which is called absolute zero, the movement of atoms and molecules slows down to a complete standstill.