Key Fobs

The remote key fob for a modern automobile is a pretty cool little device.  It contains a pseudo-random number generator, that synchronizes with your car every time you press the button.  And the system will store 256 additional numbers, so if you press the button when you're out of range, it'll recognize the next number in line, and promptly resynch (if you manage to press the button 300 times in a row when you're out of range, check the owner's manual-- there'll be a way to force a resynchronization).  The fob uses a 40-bit code, which means roughly 1 trillion possible total codes, divided by the 250 that it stores (plus multiple possible fobs per car) gives you still roughly only 1 chance in a billion to accidentally open someone else's car.  This is a vast improvement over the "olden days," when (so I was told) car manufacturers only had a dozen or so basic key shapes.  I myself have accidentally unlocked and gotten into the wrong car (it wouldn't start, and I realized my mistake almost immediately).

Apparently there is a small battery in there, which will run down over time.  The fob can be opened, and there should be instructions inside the cover for replacement.  I was kind of hoping that it ran off of some sort of capacitor instead that charged when you put the key in the ignition, but I guess that wouldn't last long enough if you didn't use your car frequently.