Remington started the Morning Upside-Down-Dog thing. I don't remember when exactly it started, although I do remember that I never saw him upsidedown for any reason for months after we got him. It was very difficult to teach him to roll over because, as soon as there was the slightest danger of his belly being exposed, he'd panic and jump to his feet.. After our allegedly world-famous animal tricks trainer sort of gave up on figuring out how to get him to roll over, I worked at it one inch at a time. He was willing to lie on his side, so I'd lift one of his feet an inch or so, praise, treat, and release. Repeat ad infinitum over a period of a few weeks, gradually lifting the legs higher, then both legs, rubbing his legs and sides and tummy, until finally he was willing to expose his belly to the world, and after that we could manage the full roll-over.
Sometime after that is when he started being willing to roll around on his back to show he was a happy dog, and somewhere in there it became part of his morning wake-up routine: Groan and unfurl far enough from his curled-up position to be on one side--but with his feet all still tucked together into a tangled mass of long slender toes, long slender legs, and fur. Usually next step was to rub his face with one paw, and leave the paw resting on his face. Then unfurl completely, and finally with a bit more groaning and happy understated growling, roll fully onto his back and start wriggling around, kicking his rear legs into the air, muttering happy things under his breath. I could join in by rubbing his tummy--for which he'd freeze for the duration, then go back to upsidedown dogging--or by putting pressure against his back feet, encouraging him to kick harder and push out with more intensity.
When Jake joined the family, he'd jump up in startlement when Rem started his thing, and kinda watch and sniff. And then, not too long after he moved in, and never being one to let his one brain cell go to waste, he decided that that's how dogs in this household were supposed to start their morning, and he joined right in doing upsidedown dog. He's a little more manic than Rem--as well as digging his shoulders in and writhing, he twitches back and forth and jerks and pops, puncutated with little yelps and minibarks along with the usual grunts, groans, growls, and giggles.
Rem has cut way back on his morning upsidedown dog over the last couple of months, but it re-erupts at odd times. Last night, I went to bed a little early. Had been in bed maybe an hour when Jake remembered that he had unfinished business in the back yard, so while I was turning on the light and finding my slippers and bathrobe, Rem went through the morning wake-up routine including the full upsidedown dog whoozie. Go figure.
Tika hasn't bought into the upsidedown dog routine. She always just stretches out and crawls up alongside me. Sometimes she rolls onto her back and exposes her white furry underparts to the air for gratuitous rubbing, but I don't know that I've seen her actually rolling and growling in that happy dog manner, except for a couple of times out on the back lawn last summer.