Frustration on its own is harmless. S-4 is learning quite quickly that emotions are not as simple as Gaster's flash cards led him to believe. There's no one simple word or expression that defines them. They layer on top of each other, and it's when they layer that he really needs to worry. Frustration paired with fear, anger, sadness, or guilt is dangerous. Frustration completely on its own is a little easier to understand and handle.

So he grins back at Sans-Serif, even as he sits up, and then makes a curious face at what his brother has to say. There might not be any glue, but... it still is worth looking, especially when Sans-Serif can navigate the castle quickly. He grins and slips off the bed, offering a hand to help his brother to his feet.

"If it's not in there, maybe there's... an art room or something?"