Engineers are always fighting against friction: trying to reduce it so their machines can go faster and their parts wear down less. But imagine what would happen if friction disappeared completely. Everything would start slipping out of hands and falling off the table. Knots would unravel by themselves. But knots?! Machines wouldn't be able to move without friction! Wheels would just spin in place, not gripping the road. We wouldn't even be able to walk, because when we walk, we push off...