Eye boogers. Dream dust. Sleepies. Sleep sand. You know what I’m talking about. The gunk that collects in the corner of your eyes when you sleep.

SciShow host Michael Aranda explains just what it’s made of and how it forms in a recent installment of Quick Questions. The technical term, it turns out, is Rheum. Or Gound. Neither of which is as fun to say as eye boogers. Rheum, though, is any thin discharge that comes from micas-y parts of the body like your eyes and nose. The kind that comes from your eyes is made up of all kinds of junk, like mucus, dead skin cells, oil, dust, and bacteria. It gathers and dries into the crusty residue in the former of your eye.

Different factors impact the make up and consistency of the discharge that builds up in the corner of the eyes. Learn why some are dry and crumbly and some are wet and sticky, below. Also, when and if you should become concerned about your eye boogers.