Marine biologists have known for a while that the secret lies somewhere in the way they can alter the rigidity of their tails. The problem is that it’s difficult to measure that while a fish swims. However, using a combination of fluid dynamics and biomechanics, researchers from the University of Virginia say they’ve derived a formula that not only provides an answer to that question but also allows a robot with a specially designed tail to be nearly as good as its natural counterpart at speeding up and slowing down in water.