If you go to Starbucks, Dunkin’s or 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 won’t. It’s so common we’ve intuited this to be the right answer.

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 phenomena 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 more expensive than plastic cups. We should ask why hot drinks aren’t served in the cheaper plastic cups, and then the answer becomes more obvious: they soften and warp when exposed to heat, even as low as 60C.

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