If you've ever heard of when a dog's owner says that the tennis ball or disc (Frisbee) is slimy from play, that's because as dogs play hard, we sweat through our feet. And we regulate our body temperature through our breathing. Or panting to get that cool air in our lungs and body.
We also tend to drool and accumulate some doggie saliva which gets on the disc!
But about that spit art!
When our humans throw our disc, after several quick rounds, well, we tend to drool on the disc. And it accumulates. Most of the time you never see what does build up, but in the right light, well, we canines can't help it. It's not our fault.
There's a few more pics in my Facebook album called Dog Spit Art.