When I was a kid, I got an 3R12 (or are they called R12?) battery. I don't know why, but I tried to put my tongue so that it touched both of the little metal things, which made it tingle. It felt weird and probably was not very healthy.
I don't remember if the same thing happened when I simply touched both metal things with my finger. I have no such battery today to try it. Maybe it only is picked up by the tongue because it's more sensitive?
However, if I hold an AA battery with my fingers touching both sides, I feel nothing. Why do I not get zapped in that scenario? Does it happen, but I just don't feel anything?