The Cumulonimbus—the puffiest and presumably most glorious cloud—happens to be the ninth one mentioned in 1896’s International Cloud Atlas. It’s possible that’s where this metaphor for bliss originated. But it could also be a reference to the ninth sphere of heaven (the one closest to God) in Dante’s Divine Comedy or to the ninth stage in the Buddhist journey toward enlightenment. None of these explanations is very convincing considering that, when the phrase first appeared in 1935, it was actually “cloud eight.” Some slang experts trace the phrase back even further, to “cloud seven,” a wink to the popular 19th-century cliché “seventh heaven.” Like kindergartners arguing with “infinity plus one,” people may have boosted the number to boast about their happiness. Maybe cloud 10 is when we all stop arguing and enjoy nap time.