Well, RFID is a little more advanced than that. There's a tuned coil attached to a small chip. The toolboth has a transmitter that bombards the area with RF radiation, then listens. It does this a couple hundred times a sec. The coil, when bombarded with this radiation, stores it up then unleashes in in short bursts, translating into anywhere from 1 digit to nearly 1000 at this point in time. Some chips are actually capable of storing lists of numbers and transmitting them based on the "request" from the transmitter/reader. The Air Force uses these to track the contents of pallets for example. My dog has an RFID chip in her, so when she rode in the car thru the EZPass it never caught the pass's number. Got a ticket in the mail that took a little while to sorty outy.
Sidenote, I'm always thinking, when I got my first motorcycle the ignition switch was bad and I started thinking about rigging it to read the RFID tag in my wallet (or implanted in my ass by the local vetrenarian (that's what animal microchips are) so my bike wouldn't need a key.