Cuckoo Spit.

The stuff of childhood fantasies. Rebellious cuckoos roamed through gardens and woods, spitting on the flowers and plants with complete disregard for the niceties of civilized society. You just didn't spit if you were a nice person! As a result, whenever we touched the stuff, we would screw up our faces in disgust.

Not sure that today's kids will like the factual explanation any more.

If you want to know what it really is look here.