Friction is a force that happens when two surfaces rub against each other. It always pushes against the direction something is moving, which slows things down or keeps them from sliding. Rough surfaces create more friction than smooth ones. Without friction, you could not walk, write, or even pick up a glass of water.
Have students slide a block across different surfaces (carpet, tile, sandpaper, ice) and compare how far it goes. Rub hands together quickly to feel the heat friction creates. Race toy cars on rough vs. smooth ramps.
Imagine trying to walk across a floor covered in oil. Your feet would slip with every step because there is almost no friction between your shoes and the slippery surface. Friction is the force that resists motion when two surfaces touch and rub against each other. It is the reason you can walk, the reason a car's brakes work, and the reason a ball rolling across the grass eventually stops.
The amount of friction depends on two things: how hard the surfaces press together and how rough or smooth the surfaces are. A heavy box on a rough carpet is very hard to slide because both the weight and the roughness create a lot of friction. A light puck on smooth ice slides easily because the weight is small and ice is very smooth.
Friction always pushes in the opposite direction of motion. If you slide a book to the right across a table, friction pushes it to the left. That is why moving objects slow down and stop unless another force keeps pushing them. Friction also turns some movement energy into heat. Rub your palms together fast, and you will feel them warm up — that warmth comes from friction.
People sometimes think friction is always a problem, but life would be impossible without it. Friction lets tires grip the road, lets you hold a pencil, and lets a rock sit on a hillside without sliding away. Engineers work with friction every day — sometimes they want more of it, like on brake pads, and sometimes they want less, like inside a car engine where oil reduces friction between moving parts.