There exist two distinct modes of using rotation: first, simple rolling; and second, the use of wheels, which spin relative to a fixed axle or body. Several organisms are capable of rolling. However, despite their utility in human vehicles, true wheels do not appear to play a significant role in the movement of living things (with the notable exception of certain flagella, which function like corkscrews). Biologists have expounded on the reasons for this apparent absence of biological wheels, and wheeled creatures have appeared often in speculative fiction.