The molecules in liquids and gases are continually moving in a completely random fashion. This is known as Brownian motion, after the botanist Sir Robert Brown who first studied the nature of their movement. He demonstrated that pollen grains placed in water move erratically. This motion is due to the pollen grains unseen impact with water molecules. The tiny water molecules are able to move the much larger pollen grains because there is large number of water molecules and they are moving very fast.
Diffusion is the gradual mixing of two or more different gases or liquids. Diffusion happens when the molecule of the substances collides and intermingles. For example, the scent of flowers spreads through a room because its molecules diffuse through the air. The process of diffusion supports the idea of moving molecules, since the particle must be moving in order to mix.