In most machines, a lubricant (like oil or grease) or a bearing is used to limit friction (rubbing) between moving parts. Keeping friction down is important for limiting wear damage to the parts, and for keeping the parts from getting too hot. The faster the parts move, or the more force between them, the more important it is to reduce friction. In animal joints like the human knee, friction is reduced with a tissue called cartilage that has a low coefficient of friction, and a lubricating fluid called synovial fluid that has a low viscosity (resistance to flowing). Gerhard Scholtz, a professor at the ("Institute for Biology and Comparative Zoology") at Humboldt University of Berlin, has written that a similar lubricating fluid or material from dead cells could help an animal wheel to turn freely, but something like this has never been seen in nature.