Whenever I was younger, I used to take two 9v batteries and plug them together. Inevitably, they would both drain almost completely. Is all that energy lost to heat? (One battery would get very hot) Or is there something else here I am not getting?
It’s nearly all lost to heat, but some is used to drive the chemical reaction. You are creating a near zero resistance circuit and applying 18 volts to it. Ohm’s law tells us that this results in an infinite amount of current (18÷0). Actually the cells have some internal resistance, which increases as the chemical reaction takes place, so we’re not really dividing by zero.
If you’re lucky, some of the energy might be converted to smoke & flames.
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Thanks for the info, Opus.
Oh, and not to worry–As I said, that was a childhood hobby. Now I’ve moved on to bigger and better things, like using the hairdryer in the shower and peeing into electrical outlets.
But, thinking some more, I have another question. Why is it that only one of the batteries would get hot? Even set on their sides (to negate gravity, not that I’m even sure that it would be a factor here) why wouldn’t they both be the same temperature? Why one hot and one cool?
Easy. One battery was Yin, one was Yang. 
I heard about a guy who put a 9v battery in his pocket after a few minutes, there was a “pop” and down from the sacket pocket was ejected from the side of the jacket. Apparently, there was a .22 round in his pocket and it completed the circuit. The heat built to such a point that the powder ignited.
I read about a pilot who put a 9v battery into his pocket, which also contained an aluminized wrapper from some (Lifesavers?) candy. He said it became quite hot and could have cause a fatal distraction in a cockpit.
UL, or no?
I’ve done this with coins in my pocket. After a time, it did get quite warm, but not to a painful level. Once with a fairly new battery, I took it out when I noticed the heat. After holding it for a few seconds, it was uncomfortable to a barely painful level.
I don’t think this would quite qualify as a fatal distraction. Sneezing is more distracting.
The hot one has a higher internal resistance than the other. Perhaps it was older, or more used up. Most of the 18 volts were across the hot battery, and less acroos the cooler one.
That’s also why you’re told never to mix old batteries with fresh ones. Depending on the wiring in the appliance, old cells may place an additional load on the new ones as they attempt to “recharge” from them. Conventional dry cells cannot be recharged in this manner, but they’ll die trying (and wear out the good ones in the process). Worse, the bad cell might leak into your Sony Walkman, voiding your warranty.
We’re just looking out for you boneheaded consumers 
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Just in case you ever wanted to know what would happen . . .
Placing the terminals of a 9v battery on your braces is very painful. You will be knocked backwards out of your chair and you will lose your vision for a few seconds, and see stars. It feels roughly equivalent to getting hit in the head with a fast-pitch baseball or running (as opposed to walking) into a metal beam.
I had a mildly painful childhood.
One time I put a 9-volt in the same pocket I carried my pocketknife in and it got my attention rather quickly. (This is also the genesis of an old “close your eyes and stick out your tongue” practical joke.)
Seriously, though, I’m trying to remember the last time I saw something that needed a 9-volt. Since Walkmans came out using one or two AAs (replacing the older 9-volt driven transistor radios) has there been a movement away from 9-volts?
And as soon as the “Thank you for your reply” came up, I remembered one thing: smoke detectors. But is there much else?
Well, at least I got to pad my post count…
Among my stuff: moke detector, and an electronic guitar tuner (made by Korg, IIRC).
When a dry cell battery is short-circuited as previously described, the internal resistance rises far faster than would be the case in a rated delivery of current.
If you did the calculations about how much energy was released you would find that this is much less than the stored energy in the battery.
The reason is that when a cell is discharging the plates become contaminated, which is what causes the internal resistance to rise.To counter this in normal use chemical depolarisers are employed within the cell construction.
The speed of short-cicuit discharge is such that the depolarisers are unable to decontaminate the electrodes quickly enough.This is designed into most dry cells as a safety feature.
Don’t try it with rechargables-the internal resistance is much lower and the process of polarisation is differant and not enough to prevent possibly disastrous results
Don’t forget the DOD distortion pedal. I think ALL pedals I’ve ever seen use 9v cells.
I’ve repaired many a DOD dist pedal and overdrives too. give me that valve sound any day!
But my alarm clock takes a 9V as a battery backup.
The semiconductor used in the IC will make a big difference as to what battery is used. Most of the new electronics out there work at under 6 volts. Cmos IC’s and transistors worked at higher voltages. It’s the IC requirements that most electronics are built around.
Now touch that 9 volt to your tounge and see if it’s still good.
Okay, boys and girls, here is the absolute best use of a 9 volt battery: you can start a fire in a just a few seconds with a 9 volt battery and some steel wool. I have a kit in my back pack that contains a small swatch of fine mesh steel wool in a ziplock bag and a 9 volt battery in another ziplock bag (together in an “Old Blue Sock” bag). Be sure to keep the little plastic protector on the terminals of the battery (use electrical tape to keep it in place) until you are ready to start a fire. Just touch both terminals to a bit of steel wool and you will see the steel wool begin to glow red hot. Add a little fuel (twigs, grass, pine needles, paper, dried moose nuggets, camping permit, etc.), and - voila - you’ll have Smokey The Bear paying you a visit!
Hopefully this post maintains the spirit of the title of the OP.
When I was a kid, I really liked putting a nine-volt on my tongue. The sensation was “zippy-metallic”, if that means anything (it’s the only way I can describe it). I don’t believe that it is a dangerous activity, but don’t blame me if you try it out and you explode.
Maybe “stupid things I used to do as a kid” ought to be a MPSIMS topic. note to self: use search engine first!