* Death by Glamour
Frisk takes matters into their own hands

Notice, he is a freaking giant compared to the humans

Papyrus, Undyne and Toriel are taller than Asgore, who we’ve already established to be A FREAKING GIANT COMPARED TO HUMANS
But let’s focus on Sans.

So, then, Sans, compared to the adult humans, would be…

Look at this, his head is half the size of the human

Now, how big would Frisk be in comparison, then? Depending on Frisk’s age, he/she/they/zir/whichever-you-use could have been one of the following heights

Holy. Fricking. Frickity. Frick.
That would explain why Papyrus saw the rock before he saw Frisk. Frisk was probably so tiny, he escaped Papyrus’s attention.
I MEAN LOOK AT THIS!

They are all huge!!!! Frisk is like a kitten compared to their heights!