I used to rub the side of my nose briefly to make fingerprint reader work on thinkpads, I think this coats the finger with enough oil to make it work reliably.

It works for my phone's fingerprint scanner. I used to have issues with it, and eventually thought it might be caused by the extra "safety glass" glued on top of the screen. Then one day, after another failed fingerprint unlock attempt, I noticed a text on the screen suggesting to moisten my finger. It must have been added in some system update, and I'm very thankful for that, or else I'd have to wait until this HN thread to figure this out.