Beware of pots, glasses and bowls that are the same size, or close to it, and especially double-split saucepans. If you stack them in the sink with one on top of the other, they can get stuck together, and while it seems like it should be easy to unstick a pan, it often isn't. The combination of thermal expansion caused by hot food or liquid, the presence of something sticky and the inability for air to pass the near-perfect seal around the rim create a gnarly situation that could result in something getting broken if you use too much force.
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Why Pans and Dishes Stick
When a pair of pots, pans, dishes or glasses are almost the same size, and you put one inside the other, there's already a natural frictional force that makes them difficult to separate. Add heat, and the resultant thermal expansion of the vessel materials completely closes the gap between the two vessels to the point that air can't pass to equalize pressure in the lower vessel, and now you have to also overcome a suction force to separate them.
Of course, the binding force holding the two vessels together could be something simpler, such as a sticky or hardened coating on the inside of the lower vessel. It could be caramelized sugar left over from cooking or solidified honey. This is a distinct possibility if you have two glass bowls stuck together, and separating them calls for a different strategy than you would use to separate a double-split saucepan in which you were boiling water.
Unstick Pan from Another Pan
If two saucepans stick together while you're cooking or immediately after you remove them from the stove, the strategy for unsticking them is to reduce the temperature. The Hometalk forum offers several possible ways to do this, including setting the lower pan in hot water and putting ice in the top pan, leaving both pans in the freezer for five minutes and giving the pots a quick twist.
You probably already tried twisting the pots to no avail. The other two suggestions will probably be more effective, but there's an easier way: Hold the pots under cold water. Let the water run from the faucet until it's as cold as it's going to get, let it run over the sides of the pots for about a minute, and they will eventually come apart as the metal contracts.
People have been turning to The Old Farmer's Almanac for solutions to household quandaries such as this for over 200 years, and it recommends dribbling white vinegar or warm vegetable oil around the rim to reduce friction. Try this if the cold water method doesn't work.
Separating Glasses and Glass Bowls
When glasses and glass bowls are stuck together, it's often some sticky substance that is binding them, and instead of cold water, you need hot water to separate them. Let the water run from the faucet until it's as hot as it's going to get, then hold the vessels sideways under the water for about 30 seconds. That should be enough time for the heat to soften or melt whatever it is that's holding them together, and you should be able to pull them apart.
If that doesn't work, let the water temperature change slowly until it's cold and try again. Don't change the temperature abruptly, though, or the glass could crack.