Where does the term "Say Cheese" come from when taking portrait pictures?
I can understand instructing people to smile, but why "Cheese"?
I believe the use of "cheese" in the phrase is because of the position it puts the subject's mouth in. Mouth the word "cheese" slowly and you will notice that the ending "e" or "uh" sound puts the corners of your mouth in a smile.
People like their portrait when they are depicted smiling. Uttering words with the long ‘e’ sound does the trick. Watch the birdie and booby and cheese forces us to show our teeth with a smile.