Every since the boys were able to hold their heads up, I have bounced them on my knees/given them airplane rides on my feet.
Without realizing it I started making a noise that sounds something like, "Boop-a-dee Boop-a-dee Boop Boop Boop Boopie Boopie Boop Boop Boop."
I didn't realize it until Peter came up to me one day asking me to do "Poopies" for him.
Thankfully, he promptly climbed onto my legs after requesting, so the request was easily understood.
Since then we have explained that he needs to ask for Boopies - with a B.
This past summer my cousin Sam took a video of this game: