Leo made it all the way through our cavernous new Costco without much fuss over the baguette he requested and which I placed in our cart. He so desperately wanted to attack it and shove it in his mouth while we wound our way through the aisles between the bakery section and checkout! But I told him he would have to wait, and that he could have a bite once we got back to our car. He made a few more requests for his bread and hit his head once or twice along the way, but was easily calmed, and -- so you know our benchmark -- I rate one or two yells/head slaps strictly mathematically, i.e., almost zero.
His tolerance of delayed gratification was amazing. It was not so long ago that I would have let him have a few nibbles so we could complete our trip. Now I have faith in him, and his self-control.
I tore him a hunk of baguette once he'd strapped himself into his car seat, then started loading our haul into the car. His sisters noticed him nomming the baguette, and pleaded for some, too. So I tried an experiment: I tore another piece from the baguette, then handed it to Leo, and said, "Leo, pass this to Izzy."
He did. Though all he wanted to do was to eat the bread himself, and without me mentioning any kind of reward, he did what I asked.
That would be a milestone.