When moving in water, rotating mechanisms are more efficient only when the "Reynolds number" of the flow is very low. The Reynolds number is a way to describe the behavior of a flow—in a flow with a low Reynolds number, like those in which bacteria move, viscosity is the most important effect to consider. In flows with high Reynolds numbers, inertia is the most important effect, and mechanisms that "oscillate", or move back and forth, are more efficient. Although ship propellers usually have an efficiency near 60%, and aircraft propellers around 80% (up to 88% in the human-powered "Gossamer Condor"), oscillating foils like fish tails and bird wings can be as efficient as 96%−98%.