rain 1. n. liquid water that falls from the sky; 2. v. to fall from the sky as water.
reign 1. v. to rule over a kingdom; 2. n. a period of time during which a particular ruler or family controls a kingdom.
rein 1. n. a usually leather strap attached to a horse or other domesticate and used by a rider to guide its movements. 2. v. to manage or control a willful person or creature, usually with in (~ in).
Words in the Wild: He knew if he didn’t rein Gail in she’d become stormier than the rain thrashing down around them—there would be no telling when her reign of terror might end.
Of these terms, it’s most often rein and reign that I see confused in the work I edit, usually in the turns of phrase to rein someone in and to give someone free rein. Both of these metaphors compare unruly minds to unruly beasts, so rein is the word you want.
Still vexed? You can find a complete list of the Word Blog’s lexical vexations here.