system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that paper cups get mushy and starts to fall apart within an hour when holding cold fluids, but are perfectly okay with holding hot fluids like coffee indefinetly?

The Starbucks I go to ran out of cold cups for my ice coffee. They put in a hot cup instead. In a matter of a car ride,  it almost fell apart as I picked it up.  This never happens with my hot coffee?

Edit: i get it, I get it. It sounds like my coffee is hot indefinetly.
gpt
Paper cups are coated with a waterproof layer on the inside, but not on the outside. When they contain hot drinks, the liquid is touching this impermeable barrier, so doesn't get the paper wet. However, a cold drink cools its container, making water vapour from the atmosphere condense on the outside - that's why your ice water glass "sweats" on a hot day: the water forming on the outside is not seeping through the glass, but rather condensing out of the air, just like when you breathe on a mirror and it fogs up. 

However, the paper cup only has a waterproof barrier on the inside. This condensation on the outside of the cup can soak into the paper and make it go soggy.