MYSTERY LIQUID?

I have been walking around my house for the past couple of days finding these little puddles. They look kind of weird, sort of gross. I guess they are just gross because I can't figure out what in the world they are. It isn't water or juice dripped from a sippy cup, it's not milk from a bottle, dog slobber??? The puddles have these little bubbles in them? I haven't asked anyone in the family if they know what it is because I'm half thinking I'm going crazy. I find them in the weirdest places. And then tonight we walk into Favorite Boy's bedroom after his bath. He stops dead in his tracks in the middle of his room to pick up his step stool and says, "I better spit on this" !!!!???!!!! And he spits the biggest pool of spittle right smack dab in the middle of the step stool. I've never seen him do this? I've never heard of him doing this? I've never witnessed him watch someone else do this, and especially not INSIDE my house! So my reaction is, (imagine this very dramatic exasperated reaction because it was) "Favorite Boy why would you spit and why did you do it in the house? We don't do that!" And his logical three-year old response is, "um, because I wanted to" I still have no explanation of where he learned this nice little habit but all I could do was run out of his room as fast as I could (to giggle) because I know for 100% certainty that my mom wished this upon me!! I remember her bopping my brother upside his head on an hourly basis all summer for about ten years because he spit every where he went. I'm pretty sure his response to our mom was the exact same as Favorite Boys was to me.