The fob that you carry on your keychain or use to open the garage door is actually a smallradio transmitter. When you push a button on the fob, you turn on the transmitter and it sends a code to the receiver (either in the car or in the garage). Inside the car or garage is a radio receiver tuned to the frequency that the transmitter is using (300 or 400 MHz is typical for modern systems). The transmitter is similar to the one in a radio-controlled toy. See How Radio Works for details on radio waves and radio transmitters.