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.