It was established in an earlier strip that he does that – however, I’ve seen a few strips where he’s sleeping in the Baby Bjorn (or whatever carrier Mom uses) and his eyes were closed.
I used to do that a lot as child too! The key was feeling the pressure of your body on the ground over your entire back surface. Strange that the feeling of solid, motionless connection was also what enabled the feeling of the motion. Don’t recall ever pretending to be a starfish though.
A comment I never expected to be making – I love the drawing of the school bus pulling up fast on a dreary November morning. Will has captured the mingled dust and leaves and puddles of dirty water I remember so well from my childhood. He always excels at these quotidian things that hook the memory, like the back of the Snack Shack or Wallace’s dad’s toolshed/workshop. I say again, nobody else is doing this kind of work in comic strips today.
I also thought I remembered that call from an earlier Wallace strip, but, like you, can’t remember when it appeared. It does sound familiar though!