When we feel cold, we like to rub our hands together, the faster that we rub them, the warmer that they get. This action is called friction. In physics, "friction is a force that opposes the motion of an object when the object is in contact with another object or surface".

There is no such thing as a totally smooth surface. Even if they look smooth to the naked eyes, if we look at them through the microscope we will see that they have ridges and grooves. Friction occurs when the "ridges or grooves of two different surfaces contact each other as they slide past each other".

There are different kinds of friction:

Static Friction: "the force that keeps an object that is not moving in its place". A vase on top of a table is an example of static friction.

Sliding Friction: "the force that occurs when two objects slide against each other". An example of sliding friction is striking a match against a rough surface.

Rolling Friction: "slows down the motion of an object rolling on a surface". Rolling friction slows down the motion of a tire rolling along the pavement.

Fluid Friction: "the force that slows down an object when it travels through a fluid". A fish swimming in the water is an example of fluid friction.