Maybe most people don't, but skeletons are outliers. They might ask Frisk strange questions, in the right circumstances. Dog marriage and all.

"Oh! So it doesn't," he notes, accepting Frisk's implied explanation. "Doesn't that get inconvenient? Needing different clothes for different seasons?" It's a very good thing they're near an ocean; Undyne would be unhappy at both ends of the year, especially the summer, if she didn't have a good place to go cool down.