You should write each source comment and commit message as if it were your last. In other words, if you died and weren't around to answer questions, then someone else should be able to figure out why in the world you wrote that code.
“And you can’t go to the royal easter show without trying a Dagwood Dog. We’re hoping the royal couple will take the time to chow down on one of those today.”
*Vomits in more ways than have previously been known to medical science* #nt