Cody and I have played this game since he first started smiling and giggling. I would lay on my back and hold him above me and tip him a little so he would be slightly upside down. Then I would say "Upside down baby (eeee)" in a sign song voice as I was flipping him slowly back up. We still do this from time to time. But now I think he has found he enjoys standing on his head. Last night when he shoulda been laying down he was doing this.
He does this a lot.
Maybe his breathing treatments get to him. Haha. On a side note, his mommy use to love to stand on her head. Silly, but true. Betcha didn't know that about me.