The biggest mysteries about what Undertale’s gender-ambiguous protagonist has in their pants are:
1. How the pie, hot dogs, and other unwrapped food items in their pockets don’t get mushed together into disgusting mutant goo.
2. How the pie’s plate fits AT ALL.
3. How EVERYTHING fits AT ALL.
4. How the snowman piece, bisicle, and Nice Cream don’t melt. (MAGIC.)
5. How Frisk knew that cramming these items into their pockets was a good idea in the first place. I mean, Toriel implies that that’s where the kid keeps their stuff, but when have you ever looked at a piece of pie and thought “That should go in my pocket”?
Did they shove their hand into their pocket, feel the presence of this newfound pocket dimension in their pants, shrug, and say, “Welp, in a world with ambulatory skeletons, talking rocks, and murderous flowers, this is a normal level of weird, so I might as well just roll with it”?
And of course, there is always the most important question of all...
6. Do they have enough healing items to get through the next boss battle?