So, you have a friend or family member who's a math and/or memory geek, but you're not sure what to get them for the holidays? Not to worry, the 2005 Grey Matters Gift Guide is here!

What better place to start than a T-shirt featuring a mnemonic on it? This T-shirt from ThinkGeek.com features the classic mnemonic for the formula for sulfuric acid:

Johnny was a chemist's son,
But Johnny is no more.
What Johnny thought was H₂O,
was H₂SO₄!

Of course, where math and memory skills meet, Pi is always there! For those who enjoy the classic challenge of Pi, you can encourage their achievements with a T-shirt featuring 4,493 digits of Pi! Don't forget, though, that there are ladies out there who take on the challenge. For them, there's the same t-shirt design available in babydoll style.

I can here some of you saying, "Sure, T-shirts are a great Christmas gift in some places, but what about those of us in the northern hemisphere?" No problem! Just wrap yourself in a nice, warm blanket, featuring the first 410 digits of Pi. I'll even help you memorize 400 digits of Pi at no extra charge. You're on your own for the remaining digits, however.

Before we leave the Pi-related gifts, why not tell the world how easy memorizing Pi can be, with an "Simple As 3.141592..." T-shirt?

That last T-shirt is put out by the good people at mental floss magazine (yes, I meant to type the magazine name entirely in lower case). Mental floss's motto is "::Feel Smart Again::". Although it is one of the more inexpensive gifts in this guide, a mental floss subscription can help your favorite geek or trivia lover feel smart all year (assuming you're not trying to avoid that)!

Speaking of reading material, there is a great selection of Memory & Mnemonics ebooks from Lybrary.com, including the hard-to-find "How To Develop A Perfect Memory", about which I wrote in my second blog post.

If your favorite geek leans more towards math, then the Martin Gardner's Mathematical Games CD-ROM, containing all 30 years of the mathematical master's Scientific American columns should be just the thing.

For someone who likes to perform the calendar recall feat, but never seems to have a portable perpetual calendar around for verification, perhaps a Calendar Wheel, or a nice wooden perpetal calendar from Sculptures-Jeux would come in handy.

But for something really special, let's consider the Knight's Tour. The mathematicians can do the Knight's Tour their way, and the memorizers can do the Knight's Tour in a completely different way, but they both still need a way to demonstrate the feat. To answer that want (need?), we have the Knight's Tour Excalibur by Devin Knight, complete with 3 different size chessboards for different audience sizes, as well as plenty of extras. For those who would like to be able to do the Knight's Tour, but don't want to put in the work, it even includes a method that makes performing the Knight's Tour much simpler, yet doesn't interfere if you wish to perform the legitimate versions!

I'm just doing my part to make your holidays happier and geekier.