You blinked heavily and sat up, frowning. Sans was snoring lightly next to you, his fingers still intertwined with yours. You bit your lip and stared at his chest. Was he really completely a skeleton....?
You gently reached out and grabbed the hem of his white shirt.
A hand clamped over your wrist and you squeaked. Sans was staring lazily at you with one eye open.

"Hey kid. How'd yah sleep?"

You blushed heavily. "I uh, em, well, it was.... peacefull."

He nodded. "No nightmares?"

You shrugged and nodded. "None. Speaking of which, I don't think I'm the only one."

Sans flinched slightly. "Yeah. I guess we do have one thing in common..."

"Night terrors." You nodded. "But why do you have them?"

Sans sat up and released your wrist. "I'm the only one that remembers all those time lines. Which means I'm just waiting for the kid- The other one- to make their move. I'm ready this time."

"Pap, we're home!" You yelled.

He didn't answer.
Sans paused, then said," he's in the kitchen. If he's quiet, he's stuck on something. Go help him please."
Then he ran upstairs.

You sighed and went into the kitchen.
Pap was staring at his pot. It was actually boiling over.

"Uh, is that safe?" You poked his shoulder.

He jumped and glanced at you. "YOUR BACK. I'M OUT OF GLITTER."

"What."

"GLITTER. I NEED IT FOR MAH SPEGHETTI." He grabbed the pot and dropped it in the sink, then got another. "WOULD YOU LIKE TO HELP ME WITH THE SAUCE? JUST DESTROY THOSE TOMATOES AND WE'LL BE GOOD." He filled the pot with water and slammed it on the stove.

You glanced at the tomatoes, then frowned. "How?"

"TREAT THEM LIKE THE ENEMY. THAT'S WHAT UNDYNE ALWAYS SAYS."

You did. You hesitated, but you did.
You and Papyrus made a huge mess, and when you guys finally paused, you looked at him. He had a lot of sauce in his face, and you giggled.

"Here, let me get that off."
You reached up, but as you did Papyrus licked it off with his tongue.
You stared at him, stunned, and took a step back.

He nyeh-ed and said,"I AM QUITE CAPABLE OF DOING IT MYSELF HUMAN."

You blinked slowly. "Papyrus, you have a...."

"....A TOUNGE? YES HUMAN, AND SO DO YOU. WHY IS THIS A STRANGE THING?"

"... Your a skeleton." You said blankly. "How do you have a tounge."

He laughed. "HOW ELSE WOULD I EAT?" He stuck his tounge out. "ALMOST ALL THE MONSTERS HAVE THESE. IT'S NORMAL, RIGHT?"

".... It's really cool, actually. You should go show Sans." Sans of course already knew, but maybe he also had one...?

He lowered his voice at the mention of his brothers name, and got really serious. "Human, I would very much like to talk to you about him."

You swallowed hard. Papyrus never acted like this. He didn't seem as happy as usual. "Y-yeah?"

"Sans has not been himself since you got here. I am quite worried. He has these nightmares, you see. Night terrors." He paused and looked at the stairs. "And he.... well, talks in his sleep."

You leaned closer. "What does he say?"

Papyrus hesitated. "I don't mean to gossip, but I believe he is not being honest with me. He talks about... well, names. He says names I faintly recognize, like I should know them but don't."

"Paps, what does he say?" You whispered.

He paused again. Then whispered, "Gaster."

".... Also he says ketchup a lot."

An:
Sans never directly answered why he has night terrors O.o
Slick.