The “ou” spelling (loud, gout, rough) is more common than “uo,” especially when the combination forms just one syllable. Where this combination does appear, we sometimes struggle with the spelling.
If you tend to consult your toothpaste tube every time you have to spell “fluoride,” it might help to know that most words that start “flu” are related to the concept of “flow.” “Fluoride” comes from the element “fluorine,” from the Latin “fluor,” meaning “flowing.” “Fluor” was first applied to minerals useful as fluxes, a term familiar to anyone who has soldered a copper pipe. Flux (related to “fluctuation”) comes from the past participle form of the Latin “fluere,” which means “to flow.” The look of fluor-type minerals when exposed to ultraviolet light gives us the word “fluorescent.”
So, if you can associate the thing you find in toothpaste with the word “flux” (things may be “in flux,” a “flux capacitor” powered the time machine in “Back to the Future”), that might help you remember the spelling of “fluoride.” And “fluorescent.”
“Flu,” by the way, also related to “flow.” “Flu” is a shortening of “influenza,” which comes from “influence,” which had to do with astrology, or the effects that flowed from the stars.
I’m tempted to keep exploring the pathways I keep uncovering on this etymological journey, but I think it’s time I got back to work.