What would you do with a single use portal gun?

Make a drain hole at the bottom of a lake, that comes out above it. Put in a water wheel, free energy.
- Never thought about it before, but I guess this makes portal guns impossible, since scenarios like this break conservation of energy.
  
  But you know it’s possible from firsthand experience. Whomsth’n’t reached terminal velocity by shooting the ceiling & floor and jumping in?
  
  Well, the videogame kind are definitely impossible, but if the gravitational field could travel through the portal then it would probably still conserve energy. The gravitational interactions around vertical portals would be exceptionally weird. If they were close enough, you'd probably experience weightlessness while in between them, but I can't wrap my head around what would happen as they move further apart. That makes me hope someone tries to make a mod that models that in Portal...
  
  Maybe conservation of energy is wrong.
  
  Maybe entering the portal takes as much energy as it would take to climb the long way? If the other end is on the moon you have to enter at 10km/s or something else you fall right back out. Warning: I am not responsible for damage caused by extreme tidal forces.
- The trick would be finding the tallest vertical line of sight you can. Just making portable plates and flying one would be hard to beat, but failing that maybe a mineshaft.
  Does it fire through water in the game?
  
  No. The only water in the game is toxic (i.e. you die if you fall in) and you can't fire the gun through it.
  
  There are no bodies of water but you can shoot through a falling stream of it.

- that looks just like me lol
- I mean, genuinely, this would be infinite energy. I was thinking of somehow using magnets and induction to generate power, but it would be excessively high frequency and would be antithetical to multi-phase power.
- See, I don't think water is the way to do it. I think you do it with a massive lead pole with teeth to lock into a gear.
  Fully contained, no spillage
- Part of me is thinking: "But doesn't the water slow down each time it comes into contact with the cog?"
  And the answer is the yes, but the answer is also, "it begins to accelerate again right after it falls off the cog."
  And yet it's still fucking with me, and somehow I'm imagining water getting slower and slower and less energetic each time, as if gravity itself is being weakened

Moon.
- How fast would this suck up all of our atmosphere? Hours? Days?
  
  Probably years, if not more. XKCD's "What If?" has a nice explanation on a similar question, but in that case it was a portal at the bottom of the mariana trench and the question was about draining the oceans. The answer is that you probably wouldn't notice in all your lifetime.
  
  Unless the pressure differential causes it to escape the moons gravity well it would probably stabilize at some point before dumping the whole atmosphere, right?
  
  Pretty sure the game theorists channel on youtube did that one several years back. It's been a minute since I watched it but a search should pull it for you.
  
  Easy solution for that, you fire the earth portal on a wall inside some sort of specially constructed airlock.
  That hard part would be getting the moon portal in a useful place, you'd probably need to go up there and find a good solid cliff face or something. On the plus side having done that you could be home again in a few seconds.
  
  As long as I'm the first one through, the rest is set dressing.

One at the white house, one at the sun.
- Be carefully secret service is now on to you. Maybe add /s so they know you're joking and not intending any harm to dumb animals there
  
  He meant to say "In portal"

I'm going to be really sad when I fire my first shot from my single use portal gun only to find out the surface needed to be coated with moon dust.

I’ve always wanted to fuck my own ass.
- Won't work, as you thrust your ass will pull away. Only way I could think of is having the portals on platforms that would move with you.
  
  I appreciate your in depth analysis of this situation.
  
  Or potentially, you could just move one of the portals relative to the other.
  
  Maybe it would just double the friction.

fuck it up and waste the first shot on a non-portal supporting surface

Put the other portal in a geostationary space station: free access to orbit.
The single most (energy) expensive thing humans do is put things in orbit.
- Love the idea, but (spoiler for the end of Portal 2) >!isn't that kind of what Chell did at the end of Portal 2 to defeat GlaDOS? The portal on the Moon causes the portal on Earth to suction everything out into space.!<
  
  Your spoiler tag doesn't work
  
  The portals would just need to be in a pressure equalizing chamber for that.
  
  You put it inside a space station... Or out the earth side portal in an airlock.
  Chell put it on the surface of the Moon.

Like, I can only shoot two portals ever? Single-use would imply only being able to shoot one portal, but that doesn't seem very useful.
Also wasn't portal travel hella toxic? To the point that it killed Cave Johnson? I sure as hell won't be using it to save time on my commute or anything.
I think I'd have to use it for violence, like sending a war criminal off a large building, but you couldn't even really have fun with it. Everyone fantasizes about launching a billionaire straight into space, but you'd need multiple portals to get them into a situation where they're building momentum first.
Also, if there's no material emancipation grids, are the portals just there forever? The portal gun doesn't have an off switch or a way to deactivate existing portals.
- Portal travel isn't toxic. However, ground up moon rocks, which are an almost perfect portal conductor, are very bad for you.
  
  "The bean counters told me we literally didn't have $7 to buy moon rocks with, let alone $70 million. Bought 'em anyway! Ground them up, mixed them into a gel. And guess what? Ground up moon rocks are pure poison!"
  
  The dust is actually really sharp irl, afaik. It never has been ground down by weather or wind. So it actually is terrible for equipment and people, lol.
- I didn't think it was portal travel that killed him, it was breathing in the moon dust

Take advantage of the free energy and build a power plant around it.
- "You're given a money printing machine. What do you do?" "Uh, print some really cool art?"

Best use would be to get two steel plates or something, of apropriate dimensions for portals and shoot them there. Now I have two portals I can move as I please and where I please
- In the game if a wall moved the portal would evaporate.
  
  Except that one room.
  
  https://youtube.com/watch?v=OrAHvenjZpA
  
  Well then I fucked up I guess :D don't remember this detail
  
  In the Valve-developed Sixense Razer Hydra expansion, you could smoothly glide a portal along a wall. Not sure if that makes much of a difference.
- Smartest answer by far.
- How thick are the plates? Are they bendable? What happens if you bend one of the portals? What happens if an object goes through a bent portal?
  o_O
  
  I imagined it being thick enoug to not bend easily. But I guess if it could, either the portal would somehow prevent it or expire

I would think about buying a house out in the middle of nowhere and link up a closet with one in my house. Free extra square feet in my house.
Realistically, I'd link up my house with a family member's house that we need to fly to.
- Your houses would become an extremely valuable trade route.

But two sheets of plywood
Portal on each
???
Profit

One on my butt, one in the toilet, never need to poop again!
Bonus points if someone finds the toilet 😉
- I mean you would still need to poop. But you wouldn't have to worry about the time or your situation.
- You're pooping on the oval office's desk arent you

I would contact NASA and tell them to launch a robot to mars that can shoot it. I'd shoot it at the ground, then put it on the robot, then have a portal to mars.
- Watching the atmospheres of Earth and Mars try to reach equilibrium right when the portal opened up would be pretty interesting!
  
  If you set one end up in an air-tight room with an airlock, it would be just like a door from a mars station.
  For safety's sake, it might be better to put that on a space station, though. Better to wreck Mars than equalize Earth and Mars.
- I'm wondering if the people suggesting it for planetary travel have played Portal 2. It uh...it doesn't end too well when they try.
  
  Just put a door on it
- Why put one on the robot? Am I missing something?
  
  It'd be a waste of time to send a human to mars.

Shoot the portal on two moveable plates. Move the plates as needed to ship goods. For instance, I imagine making a lot of money shoving sea ice into a portal exiting Lake Powell.
- Granted. The portals immediately close when you try to move the plates.
  
  As far as I remember in Portal, the portals could move if the plates moved.

Reverse-engineer it and build an unlimited-use portal gun, of course!

Reverse engineer it

I would make one portal in Tokyo or some busy city and other in another busy city and i will sell tickets to ride between the two. Or i will put them on some sheet of something that i can move so i can change destinations but in game if i move the surface under portal it will dissapear
- I will nationalize your service for free to everyone

Assuming there is only one, sell my single use for ludicrous amounts of money to some Corp that has merchandise to move.

The good answer is to create some kind of perpetual motion machine.
The cool answer is to create some kind of roller coaster that just keeps going and going
The fun answer is to create some chaos. Put one behind a door or something and the other in some unexpected place.
- Oh, see I was thinking a portal on the ISS, but absolutely you could place one against the other, plop in a MASSIVE pole with teeth to match a gear, and just constantly generate power.

Put portals on two boards. See what happens if I put one board through the other portal.
- Minute Physics did a video on it: https://www.youtube.com/watch?v=jSMZoLjB9JE

Save it. I don't have anything currently that I would want to use a portal gun on given that I can only use it once. I might use it to get myself out of a sticky situation, or wait to see if I ever move into a relatively permanent home and I could make a portal from my house to another location for convenience.
- So it'll be just like the bag full of Maximum Heal potions when the credits roll, except I imagine you could pass it on to someone with a message not to wait for the perfect moment.
  
  Maybe, but I don't particularly feel the need to use a portal gun. If I never end up using it I don't think I'd have particular regrets, but if I used it to save my ass during a high-stakes situation then I'd be quite glad.

Upon receiving it I would spend the first god knows how many hours trying to convince myself not to use it for sex things.
Beyond that, I think I'd definitely have to consider permanently renting a storage room in a country I want to visit often, and storing a portal in there.

Create infinite energy machine
- This is the correct answer given time and thought, but nah, would rather pee from my bed

Read the instructions.
Sell it to a space agency that promises it to use it as a wormhole.
It needs to be aimed at our moon (Selene) and an exo-dwarf planet.

- So one portal in an active volcano and...
  
  Ya mums butt.

I would put one portal on a pneumatically actuated plate above a dais that holds an object, and the other portal on a plate placed at a 45 degree angle. I would then see if an object on the dais would fly or fall out of the 45 degree portal, if the other portal actuated and closed the distance between its starting point and the dais.
- How would you fix evaporation I'm dumb at this stuff
  
  Anti-evaperation field, or just let rain do it's thing

Take a portal to a black hole, keep the portal open for funsies.

It would be a bad use of it, but probably pick a random road online and pick a random volcano. Whoever gets unlucky and doesn't notice the nightmare portal in the ground gets a free trip to the base of a volcano at no cost. Would obviously need the portal to be in a location where you cannot just easily return from, though. Gonna turn a random volcano and random street into quite the tourist attraction for no reason.