A mnemonic device is any technique or trick that helps you retain and retrieve information. Wikipedia has a list of mnemonics, mostly of the first-letter-based variety that we all learned a lot of in school. (Its planet ones are all based on the old classification that included Pluto; God only knows what heathen mnemonics our children are being taught in these Plutoless times.) Some of my favorites from their list:

• For the Mohs hardness scale (talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, diamond): “Tall Girls Can Fight And Other Queer Things Can Develop”
• “Fri the end of your friend” for the spelling of “friend”
• For the bones of the wrist (scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate, hamate): “Some Lovers Try Positions That They Can’t Handle”
• For the strings of a guitar: “Every Acid Dealer Gets Busted Eventually”

Another popular type involves the humble human hand. Here’s an article about the ways hand-based mnemonics have been used across cultures and time periods in subjects from music to navigation to timekeeping to religion. One hand mnemonic a lot of people learn as kids is that you can remember how many days are in each month by making two fists and holding them together. Going from left to right, each knuckle corresponds with a 31-day month and the spaces between knuckles with the shorter months.

Got any good ones? Or any really terrible ones?