Seeing the evidence

In 1828 Robert Brown used a microscope to look at a drop of water containing tiny living cells, called bacteria. He noticed that the cells jiggled all the time. This special jiggling is called Brownian motion.

Brownian motion is evidence that water is made up of particles that are always moving. They are too small to be seen even through a powerful microscope.

Some of them bump into bacteria cells and push them in different directions so that the cells jiggle. The hotter the water is the faster the water particles move about.