I'm not stranger to licking my fingers when dealing with cash, or licking stamps and envelopes, etc. but the way some old people do it was always a little mystery to me. I'd see them taking a second or three to quite conspicuously stick their tongue out and slowly lick their finger every single time before turning a page or a banknote. I always figured it's just a force of habit, but they're doing it in maximum power-save mode, and are way past giving a fuck about how gross it looks to everyone around them. I never considered that maybe they really need to do it this way to keep their fingers moist.

(That realization scares me, as it means I too might become an obnoxious finger-licker in a few years.)