One glass inside another

I would think the slightly bigger of the two. Based on my very rocky science knowledge, heat expands, so the slightly bigger glass should expand enough to free the smaller one. But that may not be accurate!

Stand them in a bowl of hot water. The outside glass will expand slightly and you will able to separate them. Good luck!

Put ice in the top glass and stand the two in a basin of hot (not boiling) water. The ice will contract the top glass and the hot water will expand the bottom one.

If we haven't used a glass decanter in a while (!) the stopper is often stuck. I just pour a little water around it and it come free. Not quite the same but its sparked a hint in me.

WD40.

Just joking!

I sometimes struggle to free the top of a new bottle of squash. I cut through the little plastic lugs to separate the cap then run it under a hot tap, after which it easily opens.

crazyH

Stand them in a bowl of hot water. The outside glass will expand slightly and you will able to separate them. Good luck!

Yes.
Don't fill the inner one! 😀

They may break, of course.
Best not to put one glass inside the other in the first place.