h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Where does navel lint come from?

And why is it always light blue?

Do you have light blue bath towels or bath robe, or wear a lot light blue sweatshirts or jumpers next to the skin.

Navel lint comes from what you wear (mostly) and is more common in people with hairy belly-buttons. Quantity also depends on how much fibre your clothes shed. Being female, I don't accumulate much (if any). My husband has more and the colour is usually dark blue or dark red, because his bathrobes are about the only things he wears that shed.

One of the few things I remember from my abortive degree in chemistry (I transferred to chemical engineering) is that there does not exist a really good black dye. Oh, sure, you can buy dye that purports to be black, and you can obviously buy black clothing... but according to our chemistry prof, what you take to be "black" in clothing is in fact a blend of various dark blues. None of the "black" dyes really stick, and they break down chemically and are diluted when they rub off the clothing.

So this stuff migrates off the textile onto the skin, and collects in the belly button. Oils on the skin surface dilute and elute the dye, effectively separating it - your belly button is doing chromatography! Anyway, the end result is although you've been wearing "black" clothes, the dye is actually blue. The lint reflects that.