To move on land, in water, or in the air, even at constant speed and at the same level, always requires an expenditure of energy. The resistance to motion that has to be overcome is of many different kinds, depending on size, speed, and the characteristics of the medium, and is a fascinating subject in itself. Even more interesting are nature's stratagems and solutions towards minimizing the effort involved in the locomotion of different types of living creatures, and man's imitations and inventions in an attempt to do as well.