How annoying is it when you get two glasses out of your cabinet and they're stuck together? After getting the dishwasher that happens more often than before because I sometimes stack the glasses before they've cooled down completely. Trying to pry them apart used to freak me out because I was worried they'd shatter in my hands.
Then I remembered this trick. Pour hot water in a bowl, put the bottom glass in the hot water. Pour cold water in the top glass.
The hot water will expand the bottom glass while the cold water will shrink the top glass leaving a tiny bit of space so you can easily take them apart. Works every time!