Oh, be a fine girl: kiss me.That sentence is a mnemonic device for remembering the spectral classes of stars: O, B, A, F, G, K, M. (Our sun is class G, a “yellow” star.)
Mnemosyne, in Greek mythology, was one of the titans, and the mother of the Muses. She represented the embodiment of memory. A mnemonic device, named for Mnemosyne (or, more accurately, her name and the word both came from the same Greek root), is something that helps us remember something else.
The acrostic is a common mnemonic device: we have a sentence wherein the initial letters of each word match the initial letters of the sequence we want to remember. Our minds generally find it easier to remember a silly, if arbitrary sentence, such as the one above, than an equally arbitrary sequence. There are lots of other mnemonic devices, of course. Acronyms are another, where we have the initial letters of the sequence put together to form a word or phrase directly. We often use these sorts of mnemonics to remember telephone numbers, making words out of the letters that go with the numbers. A friend of mine used to have a phone number ending in 3266. Damn, that was easy to remember!
Roy G. BivWhy is it any easier for us to remember that bogus “name” than it is to remember the colours in the rainbow? And what is “indigo”, anyway, apart from a mood?
Every good boy does fine.That's probably the most well known acrostic mnemonic, along with its variants (the Moody Blues used an English version, “Every Good Boy Deserves Favour”, for their 1971 album; one of my childhood schoolmates came up with “Every green bug does fart”). It's the sequence of notes, using English notation, on the lines of the treble-clef musical staff: E, G, B, D, F. The notes in the spaces form the word “FACE”, another acronym mnemonic.
Eli the ice man.In electrical engineering, this phrase helps us to remember the phase relationship between voltage (E) and current (I) in inductive circuits (L) and capacitive circuits (C): voltage leads current in an inductive circuit (E comes before I in “ELI”), and voltage lags current in a capacitive circuit (E comes after I in “ICE”). I can never remember that without the mnemonic. On the other hand, I never have to remember it, but thanks to Eli, I'll never forget it.
Bad boys rape our young girls, but violet gives willingly.OK, this one's pretty icky, but it did help me remember the colour-coded bands on electrical resistors. Black, brown, red, orange, yellow, green, blue, violet, grey, white, for the numbers 0 thru 9. The sequence also has the rainbow imbedded in it (minus that pesky indigo), another mnemonic device. One interesting thing here is that the sentence is sufficient, even though there are three “b” words and colours. I never have trouble knowing which is which among black, brown, and blue.
Be careful driving elephants into small ford garages.Here's a set that's truly obscure, and quite obsolete. The California Job Case was, in the days when printing used type slugs, a flat drawer that was used to hold the type. It had three main sections (see the rendition in the link above, or try this photo); the first two sections, which had three main rows in each, held the lower-case letters, while the third section held the upper-case letters. The upper-case letters were in alphabetical order except for J and U, which were at the end after Z. The three sentences above give the mappings for the three rows of lower-case letters (the missing letters, along with most punctuation marks and ligatures, bordered the three rows). It was very important to remember the sequence, to avoid putting slugs back in the wrong compartments — it's awfully hard to quickly tell the difference among b, d, p, and q when you look at the type slugs, so you really have to make sure they get back in the correct compartments.
Let me now help out your punctuation with <commas>.
Villains usually take <3em-spaces> and run.
Another sort of mnemonic device is the doggerel poem, and I'll leave you with a classic one, which reminds us all of the correct way to dilute concentrated sulfuric acid to avoid having it spotaneously splatter all over the hapless chemist:
If you're doin' what you ought'er
Add the acid to the water.
May your rest be long and placid
If you add water to the acid.
 It's a rhetorical question; I know that it's a sort of “blurple”.