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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?

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    $\begingroup$ Your saliva and inner tissues are more sensitive and conductive than your skin. $\endgroup$
    – DKNguyen
    Commented Feb 19, 2022 at 4:19

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with dry fingers, your skin represents a high resistance to the flow of electricity and so very little of it flows along your skin. In addition, there are no nerve endings right at the surface of the skin on your finger to respond to the (tiny!) current flowing there.

But with your tongue, it is bathed in salty saliva which is a relatively good conductor of electricity, and there are zillions of nerve endings right next to the surface of your tongue- and so even a small amount of current will zing the nerves and you will feel the tingle.

Once I was working with a 24VDC power supply with my bare hands and felt nothing- until one of the wires brushed against a fresh and bleeding paper cut on my finger. in that instant, it felt like my whole hand was on fire because blood and wet tissue are both very good conductors and the nerves nearby got seriously zapped!

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    $\begingroup$ Extending to niels nielsen's answer and adding some quantitative characteristics, the NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body's resistance to 1,000 ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms". So basically the amount of current that goes through the body is greater than 100 times. $\endgroup$
    – NMech
    Commented Feb 19, 2022 at 6:09
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    $\begingroup$ Not as significant as the tongue Vs fingers, but 3R12 batteries run at 4.5V, and AA at 1.5V, so the current is 3x higher even in equivalent conditions. (Same skin conductivity, proximity to nerve endings, and distance between + and - contacts) $\endgroup$
    – Jonathan R Swift
    Commented Feb 19, 2022 at 8:35

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