When the Grammarian Doesn't Know Everything

I hate to admit it, but I do not know everything about grammar or spelling.

I was recently trying to remember how a certain expression is spelled: Is it "rack your brain" or "wrack your brain"? Hmm. I had to look it up because I didn't know. Turns out that it's "rack."

Apparently "wrack" is similar to "wreck," and appears in the set phrase "wrack and ruin."

My valuable "Garner's Modern American Usage" tells me that "the root meaning of brain-racking refers to stretching, hence to torture by stretching." Ouch.