Friction is caused by the bumps on the surface of one object colliding with the bumps of another object's surface. All surfaces have bumps, created by their molecular structure. These bumps may only be visible on objects with seemingly smooth surfaces through the use of a microscope.
There are two types of friction: static and kinetic. Static friction occurs when objects are not moving. The resistance encountered when trying to push a heavy object across a surface is an example of static friction. Kinetic friction occurs as two objects rub against each other. Kinetic friction produces energy in the form of heat.