Locked your keys in the car, spare keys at home? Phone home on their mobile, hold your phone about a foot from the car door, get person at home press unlock button near to their mobile, car will unlock saving someone having to drive maybe hundreds of miles to rescue you.
One which does work, but is more fun than useful, is to see how much further you can activate your car's remote lock/ unlock by pointing the key fob at your head.
You lot are walking around with your mouths open, pointing key fobs at your head to get the car door open and you think I'm odd with my dowsing rods!
;-) x