There aren't a lot of T-shirts with photo geek designs on them that I'd wear in public. I've seen a lot and few are any good. Among the biggest offenders are the "Been there, shot that," "Will take pictures for food" or "Size matters" (beneath a graphic of a camera with a telephoto lens) shirts. Then there's the ever-popular graphic of a point-and-shoot on the chest, with camera straps pointing up toward the shirt's shoulders. See? I have a CAMERA. And I wear it AROUND MY NECK. And apparently it's made of CLOTH AND INK.
So I was very pleasantly surprised to get this gift: a
T-shirt with a (possibly) neutral 18% gray card on it.

Gray cards are used a couple ways in photography. When you're out shooting, you can take a meter reading off the gray card to determine your exposure. This is especially helpful if the scene is predominantly dark or light, either of which may fool your camera's built-in exposure meter. An 18% gray card will (ideally) accurately reflect the light falling on the subject and not be fooled. The other primary use for a gray card is in post-processing. During the shoot, you take a photo of the gray card itself. That way, once you're doing your digital processing in Photoshop, Lightroom or whatever, you just tell the application that the card is a neutral gray and it corrects the white balance for the photo, a series of photos or the entire take.
I have a neutral 18% gray lens cloth in my bag. Sometimes I'll throw it out into a scene or hold it up to have that neutral reference for color correction later. Or now I can shoot my shirt. Because this I will wear in public.