Fluid dynamists can steer a spacecraft through the atmosphere of Mars, but they've had a hard time explaining why the shower curtain billows inward when you turn on the water. Most blamed the motion on Bernoulli's principle, which states that the pressure of air drops as it accelerates. David Schmidt, an engineering professor at the University of Massachusetts, had his doubts. So he sat down with a $28,000 software package, created a virtual shower, and after 1.5 trillion calculations arrived at what he believes is the real answer. Everyone's been thinking about the problem backward, Schmidt says. Rather than accelerating on their way down, drops of water from the showerhead actually decelerate due to aerodynamic drag. This slowdown pulls the air around the droplets into a tornadolike vortex and creates a low-pressure center that can suck in a lightweight curtain. The process might be annoying, but we should count ourselves lucky. "In a real tornado," Schmidt says, "pressures created by the vortex can cause houses to explode."