paragraph: Feel the top of your desk. Does it look and feel smooth? Yes, it feels and looks smooth, but is it really? Even when surfaces look smooth to the unaided eye, they may not be. Scientists use a microscope to look at things more closely. Through a microscope, smooth surfaces may appear rough or bumpy. Look at the metal surfaces in Figure 1.8. The metal foil is so smooth that it is shiny. However, when highly magnified, it is clearly not smooth. It actually looks to be very bumpy. All those bumps grab the bumps of other surfaces. They grind together. They oppose the motion of the two surfaces. Now you know what causes friction. 
question: What do the bumps on metal foil do when they move across another surface?
answer: They get smooth