Rain falls from the sky.
To rein in a horse--or, by extension, one's enthusiasm--means to hold it back. Giving "free rein," on the other hand, means letting it go.
Monarchs reign from thrones.
These words are homonyms. They sound the same but have very different meanings. Since horses are no longer part of everyday life for most English speakers, there is great confusion over rein and reign. Here's another case where Matthew's recommended study of Latin would help out a lot. Journalists, take note.
[This was originally and mistakenly posted on Books in Northport. I meant to post it here.]