If you go to any large coffee chain and order a hot drink they’ll serve it in a paper cup, but if you order a cold drink they’ll serve it in plastic. It’s so common we’ve intuited this to be correct.

And on the off-chance you were served a cold drink in a paper cup, you’ll remember it being soggy. Of course, that makes sense … … wait what, that doesn’t make any sense! They’re both holding the same drink, mostly water, if anything the hotter one should seep through the cup faster thanks to more energetic molecules.

I was lucky to get served a cold drink in a paper cup today and observe this phenomenon in action. I think I figured it out: condensation. Paper cups are always lined with wax/plastic and virtually impermeable. But only on the inside. When they’re used for cold drinks water condenses on the outside and seeps into the paper, and they slowly but surely get soggy!

Well, this answer is a little backwards. Lined paper cups are about twice the price of plastic cups. We should be asking why hot drinks are not served in the cheaper plastic cups, and then the answer becomes much more obvious: plastic cups soften and warp when exposed to heat, even as low as 60°C.

So, hot paper, cold plastic. Next customer please.