The yellow boxfish may not be the sleekest fish in the sea. But despite its rectangular shape, this marine creature is a surprisingly agile 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 fold up or fan out the fin, or alter its position. 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.