One of mankind's greatest inventions is the automatic door. The idea that one once had to actually push or pull open the door to a department store now seems barbaric. But how do these marvels of modern engineering work? How does the omnipotent sliding door know that we, the humble customers, are there and require entrance to the establishment. Most automatic doors operate using a motion sensor. Motion sensors work by sending out microwaves and detecting motion using the same principle that astronomers use to determine if a galaxy is moving toward us or away from us. When door sends out microwaves, objects reflect them back to the sensor. Objects that are static send back the same wavelength as the original signal, but moving objects (like a person or stray cat) alter the wavelength through the Doppler effect. When the reflected microwaves are of a different wavelength, the door knows to open and let the customer in. This is a cool application of the Doppler effect and something to think about next time you go to Wegmans to return some stale chips.