Goo is any thick, sticky, slimy substance that clings to whatever it touches. Think of honey dripping slowly from a spoon, the way it stretches and refuses to let go. That's goo. So is the slime inside a pumpkin when you're carving it for Halloween, or the mysterious substance you find stuck under a school desk.

Goo can be natural or artificial. Tree sap is goo. So is the gunk that builds up in the corner of your eye when you sleep. Melted marshmallows become goo around a campfire. Scientists might work with chemical goo in a laboratory, while kids might make goo on purpose using glue and other ingredients for a fun, gooey science experiment.

The word sounds like what it describes: soft, mushy, and a little gross. When something is gooey, it has that sticky, semi-liquid quality. A chocolate chip cookie fresh from the oven might be deliciously gooey in the middle. But goo can also be unpleasant, like when you step in something gooey on the sidewalk and don't want to know what it is.