Rain is that wet stuff that falls out of the sky, when rainy days and Mondays really get you down.
A rider uses reins to guide a horse and to rein her in when he wants to stop.
A queen reigns over her subjects. Long may she reign! Queen Elizabeth knows how to rein in her horse as well as how to reign over her subjects. I don't know whether she rides in the rain.
This post was prompted by an online article quoting a pastor giving parental advice about a daughter: "reign [sic] her in." He was giving parents some really awful advice, like to "punch" their children, and his misuse of the word reign makes him look ignorant. Your assignment, if you choose to accept it, is to correct his spelling. In the comments, tell us whether he should have used rain, or rein, or reign. (Too bad we can't enlighten him on raising children.)