Well, we were wondering about that last night, so I figured I’d look it up… Wikipedia to the rescue.

Several different recipes of slime were used during the series’ decade-long run, some resulting in thin, watery slime and others in thick, chunky slime. In an interview with YCDTOTV.com, longtime YCDTOTV crew member Bill Buchanan explained the origin of the slime in 1979: “…one script called for this kinda disgusting slimy green stuff – but with no real indication of what it was going to be used for. … The description was that it was just something green and slimy and disgusting … Anyhow, [properties man Paul Copping] mixed up a whole green garbage can … with slime. I know he’d colored it with green latex paint. God knows what else was in it, but it was disgusting. And it was parked inside the studio door, and everyone was kinda avoiding it because it was really foul looking. I mean, he had like sausages floating in it.”

By the time it was actually used to be dumped on a child’s head, toxicity became a concern…