Lots of people these days get their exercise at gyms and health clubs, oftentimes by walking or running on treadmills--that is to say, running without going anywhere. If you've ever been on a treadmill for some time and then suddenly stepped off it, you've probably experienced the weird sensation that you are suddenly moving much faster than you think you should be. Why? This deceptive sensation is the result of sending your brain two different signals at once. The first signal is the one coming from your legs; during exercise they are in rapid, regular motion, and the brain understands by this fact that the body is moving through space. The second signal comes from the eyes, which are saying that the objects around you are not passing by, and hence you are standing still. In a sense your eyes are disagreeing with your legs. The result is that after about ten minutes of this input disparity, the brain recalibrates its internal sense of motion. In simple terms, your brain tells itself: this is how fast objects move by when I'm walking at this rate, as opposed to what was formerly the case. That means when you step off the treadmill and begin actually moving through space again, your recalibrated brain is now sent the incorrect message that you have suddenly increased your speed significantly. Once again, what the legs say and what the eyes say is out of sync. The sensation of accelerated motion is only temporary, though, and the ever-adaptable brain soon readjusts itself once more--until the next time you step up on that rolling, immobile track.