The color of sweat.

Matt:

This is probably a really weird question, but when I work up a sweat at the gym, there's always some yellowish stain around the collar of my white T-shirt. Why does the sweat make those nasty yellow stains on light-colored clothing?

-- Anonymous in East County

Weird at our house is a relative term. Sweat itself is colorless. But components of sweat, particularly fatty acids, oxidize once they soak into your T-shirt. Oxidization involves lots of electron swapping, and the resulting pit rings reflect yellowish light. Your shirts are sort of like old apples that turn brown after you bite them.