One time, when I was working as a cashier in a natural foods market, I encountered an extraordinarily rude customer. She kept demanding I work faster, and spoke to me in a condescending manner.

I could have retaliated in the usual way—press my thumbnail into a piece of fruit, crush the eggs—but then I saw something far more satisfying: her car keys hiding at the bottom of the cart. Normally, I would have just reached down and retrieved them, since they clearly fell down there by accident. Instead, I simply placed her bags on top, concealing the keys, and sent her on her way.

Moments later, I had the pleasure of watching that bitter woman frantically retrace her steps, up and down the aisles, until another employee finally discovered her keys and rescued her from her personal hell.

It was one of the most gratifying, passive-aggressive acts of revenge I've ever executed in my life. And the best part is, that ungrateful woman will never know I was the mastermind behind the whole thing.