Sometimes, a low-tech solution is all you need. If you're driving a diesel-powered car and don't want to trash the
engine by accidentally filling it with regular gas, the Diesel Guard might just be the answer. The Diesel Guard is a
$25 audio player that you mount inside your car's fuel-tank door. When you open the door, it lets off a loud beep and a
stern warning to fill the engine with diesel fuel only. We assume someone's already making plans to hack these to say
all sorts of less practical things when an unsuspecting gas-station attendant opens the door, but what we really want
is one tailored for bio-diesel cars that says, "dude, this car only runs on fry grease!"