Friction is a force that occurs when a solid object rubs against or slides past another, or when it moves through a liquid or a gas. It always acts against the direction of movement. The rougher surfaces are and the harder they press together,  the stronger the friction—but friction occurs even between  very smooth surfaces. Friction can be useful—it helps us to stand, walk, and run—but it can also be a hindrance, slowing  movement and making machines inefficient. A by-product of  friction is heat.