Everyone knows what it's like to pull out two nested glasses out of the kitchen cupboard only to find them stuck together. Here's an easy way to separate them without any risk of breaking them.
Most people simply wiggle the glasses until they slide apart, but there's always that tense moment when you're worried you might break them. America's Test Kitchen has a better way: fill the top glass with a little ice, and then put the bottom glass into a sink or bowl of warm water. Give it a second or two, and the different in temperatures will cause the bottom one to expand a little and the top one to contract a little. (This is the same principle behind why running a stuck jar lid under warm water makes it easier to open.)
It's not much movement, but it's enough to get the two apart without worrying about breaking glass.
How to Separate Stuck Glass Cups [America's Test Kitchen]