How did “cheese” become the standard verbal prompt for smiling?

It is because of the shape of the mouth. I was on a hike with an environmental group, and we were told to say trees for taking pictures.

En français, tu dis brie!

When I’m the one taking the picture, I always say, “say Mozzerel-l-la!” with an Italian accent. It always gets good smiles cause they’re not expecting it.

Apparently in Sweden, they say ost, which is their word for “cheese”, even though the mouth shape of that word is not ideal. I find that hilarious.