Uncle John knows pretty much everything—and if he doesn’t, he heads his massive research library, or puts one of his many associates on the case. So go ahead: In the comments below, ask Uncle John anything. (And if we answer your question sometime, we’ll send you a free book!)

What causes the little crusty chunks that appear around the eyes when you wake up each morning?

That gross yellow junk has a lot of different names: “crusties,” or “eye boogers,” or “sleep sand,” or “sleep gunk,” or…

Rheum is a term that refers to any and all materials expelled from the facial cavities—nose, mouth, and eyes—during sleep. The technical name for eye rheum is gound. It’s a mixture of a lot of stuff that your body made or didn’t want and then tried to get rid of when it thought you weren’t paying attention, mostly dead blood cells, dead skin cells, and dust.

That makes up the solid elements, but eye gunk is also a little wet and a little oily. The moisture comes from mucus ejected by the conjunctiva, tissue that lines the inside of the eyelids and which also covers the sclera, or white of the eye.

The oil is a substance called meibum, secreted by the meibomian glands, which line the rums of the eyelids. Their main function is to keep the eyes nearly air-tight while closed, and keep moisture inside of your eye sockets, where they can lubricate the eyeballs.

The processes carried about by the conjunctiva and meibomian glands occur all the time. When you’re awake, however, you’re blinking constantly, allowing waste products to escape your eye. But when they’re closed, all that junk has nowhere to go but the edge of the eyeball, right on the border of the inside and outside.