Seems almost criminal to put such a trivial problem in General Questions but since this has happened three times now, I figure it time I found a solution. What’s happened is this; my son was washing dishes the other day when a glass fell into a cup, and so they remain this way stuck by some form of water seal I guess. The last couple of times, I threw the pair away, but that means I’m eventually going to run out of both. I figure if there’s an answer you guys have it. As usual thanks in advance for your suggestions.
Couldn’t you just run hot water over the bottom of the cup? Air inside the cup should expand and change the pressure, right? Or am I missing something?
Avon’s Skin So Soft is phenomenal for breaking the seal between glasses, but I’d imagine anything else that would break up water tension, like maybe baby oil? would do the trick. Also, if you have a thin bladed spatula, for butter, or whatever, and can slip it between the two enough to introduce some air, that could work too.
I’d put them into a shallow pan of water on the stove and heat it up (resting upright on the cup). This will A) heat the air inside causing the pressure to rise, and B) cause the cup to expand. As the water gets hotter, at some point it should be possible to separate the two items.
If it’s really stuck by some sort of water seal, and not by the materials (are they glass?) binding up against one another, you may be able to use heat. The catch is that if you just stick them in the microwave and heat them until some of the water vaporizes, you might end up making them tighter, since the vapor will escape around the top glass, then it will fall back down so that it forms a seal again (e.g. it’s acting like a one-way valve).
To avoid this, you might try devising some way of putting them in the microwave such that they are supported by the top glass, instead of the bottom glass, then heating it. My first crazy thought is to jam the outer glass into the ring in the middle of a roll of duct tape tight enough that you can turn it upside down and the glass won’t fall out. Then set the edges of the tape roll on something so the inner glass is “hanging” facing down, and will fall out when the whole thing is heated.
Barring any working solution, I will point out that you don’t have to throw both glasses away…just break one of them and you’ll still have the other one intact. 
Rather than throw away the merged pair, wouldn’t the worst case scenario involve breaking the cup to “rescue” the glass? That’s assuming none of the suggested methods for seperation pan out.
Who is John Galt? He’s the guy who steals my ideas, that’s who!!
Stand them in a container of hot water and put some ice cubes in the glass. Forget them for a while and then just grab the glass and effortlessly take it out of the cup. Never fails.
ShibbOleth is on the right track. Put cold water in the inside cup, then lower the outside cup into a pan of warm/hot water. Google on glasses stuck together cold hot for more info.
I’m thinking microwaving duct tape is a Very. Bad. Idea.
I agree with everyone who voted for heating the outside cup and cooling the glass - however, don’t do this with ice and boiling water, as you’re likely to shatter the glass.
You guys are great, thanks.
I could be wrong here, but I thought duct tape only looked metallic. I didn’t think it actually was metallic.
Makes me want to go try it…
It’s not that duct tape will cause spectacular micro-fireworks, but it’s made of rubber and a very sticky adhesive. Both of which will melt if you microwave them.
That’s one microwave I sure as hell don’t want to get stuck cleaning.
WD40 into the seal, hot water onto the bottom, vise grips on the inner.
Failing that, a hammer.
Former professional dishwasher checking in here. Most restaurants buy glasses that can’t stack like this and get stuck, but inevitably – where 13- and 14-year-old boys are employed – this situation pops up.
As mentioned above, hot water on the outer (lower) glass should do the trick; ice cubes or cold water on the inner glass can help, but with delicate glassware you risk a thermal stress that may shatter the inner glass. Add ice cubes as you see fit, and let the assembly sit for a minute or two. While you’re waiting, get a dishcloth out.
Hold the assembly over a sink; orient it upright, hold it by the inner glass, and use your free hand to turn on the tap (hot water). Tilt the assembly so only the outer glass is under the stream, and then take a thick dishcloth in your free hand. Every few seconds, pull the assembly out of the stream of water, grip the outer glass in your dishcloth, and apply tension (pulling) first – then very gently apply torsion (twisting).
If it doesn’t come loose in three or four seconds, let go with your dishcloth hand and heat the bottom glass under the tap some more. The dishcloth does a few important things: it keeps you from burning your hand on the hot glass, it keeps you from applying too much torsion, and in the event that the torsion causes the glass to crack, it keeps you from lacerating your hand.