In the physical world, interaction without friction is nearly inconceivable. The most elaborate mechanisms would be required to pick up a bottle, set a table, drive a car, or walk to the door. Objects set in motion, whether pencils, mice, feet, automobiles, or trains, would slide freely until arrested or deflected by collision with another object. Without the damping effect of friction, we would live in an impossibly kinetic world in which the consequences of every action would persist and multiply to the point of insanity.