Diffusion

The dots represent matter particles, maybe water particles in a glass or air particles in a room.

Try to follow one particle for a while.

Individual particles wander randomly all over but the group remains fairly evenly spread throughout the space. They never significantly bunch up again.

Freely moving particles will naturally spread out to fill the available space.

The purple dots are gas particles. Break the bottle to see how the gas diffuses throughout the room.

The concentrated particles spread apart until they are evenly distributed around the space. This is a result of the random motion of the particles. It happens naturally by itself.

The blue dots are water particles. Add a drop of red food dye and watch how it diffuses through the water.

See how the random motion of particles causes the red dye particles to become fairly evenly spread throughout the container. They will never clump back together again.

Initially, the particles are concentrated on the left side of the membrane. Random motion carries some particles through the gap to the right side.

Notice that particles move both ways through the gap. At first, while the concentrations on either side are unequal, more particles move to the right than to the left. Eventually, once particles are evenly distributed, the flow is about the same each way.