Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. It is also the process by which molecules spread from areas of high concentratiion, to areas of low concentration. If you bundle a piece of retangular gold with a piece of retangular lead and give it enough time (days to months, depends on temperature, environment, etc), when you remove them you will see gold particles on the led piece and also see some led particles on the gold piece. If you want to see diffusion faster, use gas. Prepare two bottles of different gas, such as chlorine and oxygen gas, and cover both bottle with a piece of glass. Then you reverse on the the bottle, put it on top on the other bottle of gas, and then remove that piece of glass, you will see diffusion.