The yellow boxfish isn’t the sleekest fish in the sea. But its rectangular shape doesn’t stop it from being a swift swimmer! Scientists at the University of Groningen in the Netherlands recently studied how the boxfish manages to maneuver its bulky body. They placed a plastic model of a boxfish into a tank of flowing water. The model had a fanlike tail fin, just like a real boxfish. Scientists could alter the fin’s position by folding it up or fanning it out. These actions kept the fake fish stable as it “swam.” That suggests that the boxfish “uses its tail fin as a rudder to steer wherever it wants to go,” says E.J. Stamhuis, a marine zoologist who worked on the study.