Ooh, what could it be? The suspense is enough to pull his thoughts from Undyne and right back to... A window.

And outside that window? A sky. Stars.

At first, Papyrus stands utterly still. His jaw drops. His eye sockets go googly, and before long he's shouting.

"OH MY GOD IT'S THE SKY?!?!"

Give him a moment. This is. Very alarming and very much a Big Deal and those are stars? GENUINE STARS?

Pinch him. Or maybe don't. He's not sure he wants to wake up yet.