Example of diffusion of solid into solid?

Question

Example of diffusion of solid into solid?

Answer 1

http://lcvmwww.epfl.ch/~moakher/surf_dif...

Answer 2

If metallic gold and metallic lead are placed in intimate contact,  over a period of time,  molecules of lead will diffuse into the gold and vice versa.   This has been known for a long period of time.

Answer 3

when the whole spoon gets hot when it's only half in the tea.