A petard is not, as often thought, part of a ship. Rather, the petard is a small bomb, usually on the end of a stick, used to blow a hole into a wall or door. To be "hoisted by your own petard" means to be "hoisted" or thrown into the air, by your own bomb. The term gained notoriety from Shakespeare's Hamlet, which includes the line "Hoist with his own petar" in a letter from his pals Rosencrantz and Guildenstern. There's actually a fart joke in here, because petard is derived from the french word peter, which means to pass gas. When we mentioned how to protect your gas we certainly weren't talking about that kind of gas, but it did inspire this story of being hoisted by Velocitré.
The gas station I worked at in the early 70's sold these coil spring things that you were supposed to insert into your filler neck to prevent the hose from reaching the gas. Several people came in complaining that the thieves had just shoved them into the tank, and now they have to listen to them rolling around in the bottom of the tank around every corner. The boss said he'd only refund their money if they returned the item. A couple of days later his Jeep started making noise and we determined that there was about 5 pounds of assorted hardware in his gas tank. What a coincidence.
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