Diffusion in solids does not happen as in solids the molecules are too tightly packed together to be able to mix with the molecules of another solid. However, there can be diffusion between a solid and liquid. The example of an ice candy is a good one. When you pour juice over a ball of ice the juice seeps into the ice, the juice molecules mix with the molecules of ice and you get an ice candy. An example of diffusion in liquids is when you pour a few drops of ink into a glass of water. Gradually the whole water turns a light blue color. That is because the molecules of ink have diffused into the molecules of water.
Actually, diffusion *can* happen in solids, as the particles are vibrating, even if only a little. It just takes a LOT more time. But if two solids are left in contact, they will eventually begin to diffuse into each other.
The odor of perfume spreading throughout a room