When your watching Handplates and Gaster is talking about what he is going to do and explained It to the kids but you can't understand because he's using big words to scare the kids.

MY HANDPLATES THEORY
When Gaster was testing Sans and Papyrus...He tested Papyrus first to see if he could see the future. Papyrus can only see the past. Gaster then tested Sans but he lied and said he couldn't see the future. Gaster said that if he is lieing then the pain will increase every day. If Sans keeps up this act then he will be hard for Sans to stay awake. Which is where my theory comes in. If Sans does this then Gaster will ware him out and he will become the Lazy Sans we all know.

Plus Gaster only calls them 1-S and 2-S and Papyrus and Sans call each other Bro and Brother.