Friction is the force that resists motion when two surfaces rub against each other. When you slide a book across your desk, friction is what slows it down and eventually stops it. Without friction, that book would keep sliding forever, right off the edge and across the floor.

Friction happens because even smooth-looking surfaces have tiny bumps and ridges that catch on each other. Rough surfaces like sandpaper create lots of friction, while smooth surfaces like ice create very little. That's why you can glide easily on ice skates but walking on carpet takes more effort.

We depend on friction constantly. It's what keeps your shoes from slipping with every step and lets car tires grip the road. When you write with a pencil, friction between the pencil lead and paper leaves marks behind. Brakes work because friction between brake pads and wheels slows vehicles down.

But friction isn't always helpful. It wears down moving parts in machines, which is why engines need oil to reduce friction. It also wastes energy by turning motion into heat: rub your hands together quickly and you'll feel them warm up.

The word also describes tension or conflict between people or groups. When two classmates have friction between them, they're not getting along smoothly. Where there's friction in a friendship, disagreements create resistance, like tiny bumps on surfaces rubbing the wrong way.