This morning, Ann asked for my help: a wine glass had become stuck to a cupboard shelf—could I sort it out? No, I couldn't: the base of the glass really was stuck very solidly to the shelf. I even tried working the point of a knife under the glass, but it just wouldn't budge.
I asked Ann if the glass had been wet when she put it on the shelf. No it hadn't; it had been bone dry; she had taken it out of the dishwasher only last night. I asked her if the glass had still been warm from the dishwasher. Yes it had. A-ha!
I immediately deduced that, as the glass had been placed on the shelf, warm air had been trapped in the little hollow underneath. As the glass had cooled, so had the small pocket of air trapped underneath, causing the air pressure to drop slightly, thereby creating a partial vaccum that stuck the glass to the shelf like an inflexible suction cup.
So I ran the kitchen tap until it was good and hot, soaked a dish cloth in hot water, placed it over the base of the glass, and waited for the air underneath to warm up again. The glass then lifted as easy as pie.
Good to see three years of physics at university weren't entirely wasted. I'm sure there must be some money to be made on party bets involving wine glasses stuck to tables or something like that.