A pair of four-wheel skates has sixteen bearings. If they aren't working well, skating is a lot harder. If just one goes bad, it's definitely noticeable. First, some background. Ball bearings such as those used in in-lines are precisely made, but simple devices.
Ball bearings consist of polished steel balls that ride in polished ball races.
This is where the friction occurs-the balls rolling in the race. The more precisely a bearing is made, the less friction and wear occurs. But no matter how precise the bearing, metal moving against metal under pressure will result in damage to the surfaces, increasing friction. Therefore, bearings are lubricated with some type of oil or grease. The lubricant forms a thin film between the ball and race, preventing damage as they move against each other.