Friction makes it difficult to rub two dry, rough surfaces together. The thin blades of ice skates move easily on ice because there is little friction between the two smooth surfaces. Sometimes, friction can be used to make a surface smoother. For example, sandpaper is rubbed over wood to wear away the rough edges.
Friction can also wear away moving parts in a machine, eventually ruining them. To prevent this, a lubricant, such as oil, is used. Oiled door hinges will move against each other easily and there will be little wear. Some machines, like aeroplanes and cars, are also designed to reduce friction between the body and surrounding air particles.
If you magnify two surfaces which look smooth you can see that they are actually quite rough. As you rub the surfaces together they scrape against each other. Friction slows down their movement and wears them away.
Putting oil between surfaces (like the parts in a car engine) helps to make them smooth and wet. The materials can now be moved quickly and easily against each other, which reduces friction, and prevents the surfaces from being worn away.
Sports cars have a streamlined design to reduce friction between the moving vehicle and air particles.