Diffusion

When someone is cooking and you are sitting there in you living room and you suddenly feel a mouth watering smell, you know it is food. You then realise that the smell is spread all over your house. But how?
This is called diffusion. When chemicals such as the smell of your perfume or the smell of your favourite plate, are let in a room, the particles mix together(the air particles+the substance's particles).
You really don't need to shake your arms to spread the gas particles, they bounce(move around) by themselves.
When gases diffuse it is very quick, since they move quickly...

Diffusion can also happen inliquids, but it takes more time. This is because the particles in liquids can move around each other, and so, they get mixed, but since the gas particles move quicker, diffusion in liquids is slower. 

Diffusion does NOT happen in solids. This is because particles in a solid can only vibrate in it's place.