One of my favorite Word Nerd websites has a very boring name: Common Errors in English. I like that it doesn't just tell you the difference between insure and ensure; affect and effect. It tells you how important it is and who is going to care.
It says, for instance, that to ensure is to make sure something happens and to insure is to provide insurance. However, it notes, that while grammatical conservatives will get bent out of shape if you mix up the two, even though there wasn't a distinction back in the day. I'll point out before I move on, however, that it's called insurance, not ensurance; State Farm doesn't ensure your car; and it's pretty damn silly to have homophones that have the same meaning and yet do not. So maybe I picked a bad example. But I digress as usual.
Anyway, the other day while looking up beside and besides (you sit beside someone, besides means as well as), I learned the difference between rack and wrack. Nerve racked refers to being on the rack, the torture implement. Since the rack stretches folks, then to rack one's brains is to stretch them.