Washing Cups

Only... two more blogs left? *cries* How is this possible?!?! I feel like I have to think of some really awesome blog topic to sum up the year, but all I could think of was cups...

A few days ago, I was washing dishes in hot water and soap. I like to stack them in the sink, especially the cups, after I get them all sudsy. :P However, when I began to rinse them out, I realized I couldn't separate two of the cups. THEY WERE STUCK. O_o. Frantically, I tried to pull them apart, but to no avail... the cups won the battle. Defeated, I left them in the sink (like a coward) for my dad to deal with when he got home.
Later, when he came home, he asked me why I'd left two cups in the sink. He seemed puzzled when I said that I couldn't separate them, because he said that he'd simply pulled them apart.
There were two solutions to this mystery. One was that I was just extremely weak, and the other, slightly more probable one was... PHYSICS! (The answer to everything!)

When I washed the two cups with hot water, they heated up and expanded. However, the inner cup was hotter than the outer one, because I washed it later, and then the outer cup acted as an insulator, keeping the heat in. The outer cup was constantly splashed with colder water as I rinsed the other dishes, cooling it. This means the inner cup expanded more than the outer one, and got stuck inside. The surfaces were then so close together that the static friction force was greater than the force of me pulling them apart. When the cups cooled down again, as they had by the time my dad came home, they were back to their normal sizes and were easy to separate.