Particle theory helps us to explain how diffusion happens. Gases and liquids are made up of tiny particles that move about all the time, bumping into each other and changing direction. Eventually, the random movement of the particles causes them to spread out evenly. 
Particle theory can also help us to explain why bigger things - such as pollen grains - jiggle about.

Moving pollen grains

Flowers produce pollen grains to help them to reproduce.
Pollen grains are very small, but you can easily see them with a microscope. They are much, much bigger than the tiny particles that are involved in diffusion. Pollen grains cannot move by themselves. They are carried around by air currents, or on insects.
If you put some pollen grains onto a drop of water on a microscope slide, they float. When you look through the microscope, you will see that the pollen grains are moving about. They jiggle about in random directions. You will see the same effect if you sprinkle some dust on the water.
But we know that pollen grains and dust cannot swim or move by themselves. What makes the pollen grains and dust move? 
We can explain the random movement of the pollen grains and the dust using the particle theory. The drop of water is made up of millions and millions of tiny water particles. These are much too small to see. The water particles are always moving When they bump into the much bigger pollen grains or dust grains, they make them move.

A pollen grain being hit by tiny water particles that are too small to see, making it jiggle about in a random way.

Discoveries about Brownian motion

Robert Brown was a Scottish biologist. In 1827, he watched pollen grains jiggling about under the microscope. He was the first scientist to describe this motion. It was given the name Brownian motion.
Robert Brown was puzzled by what he saw. He tried to think of an explanation. He thought that the pollen grains might be moving around actively, swimming in the water.
To test his idea, he watched some flecks of dust in water. The clust moved around just like the pollen grains. Brown knew that the dust was not alive, so he knew that his first idea was wrong.
At the time that Robert Brown lived, no-one knew that substances are made up of tiny particles in constant motion, so they could not explain Brownian motion. The particle theory of matter was first put forward in the 1870s. Only then could scientists explain what was causing Brownian motion.