Why are we told to say "cheese" when photographed??

Photos are an art form of its own. Angle of taking (angle), lighting, composition, - all can determine the perfection of a photo.
But no matter how old we are, we must have heard the classic "order" when the photographer took a picture, that is "cheese!".
And, generally we respond with a big smile while saying, "chesse!".
So why are we using this phrase when taking pictures?
According to TodayIFoundOut, the idea of saying "cheese" when taking pictures first appeared around the 1940s.
The Big Spring Herald, a newspaper in Texas, printed an article referencing that phrase, in 1943.
No one knows for sure who the idea was and why the words were used.
However, one thing is certain that when saying the word we will accidentally smile.
The mentioning of the "ch" consonant makes the upper and lower teeth together, and the pronunciation of the "ee" vocal part involves lips making facial expressions that resemble a smile.
In the 19th century, only children, farmers, and drunks smiled in photographs.
Everyone keeps their face neutral, which is considered attractive and dignified at the time.
That's not the only reason people do not smile while taking pictures. At that time, it took a few hours or even days, just to take one photo.
As you can imagine, it would not be possible to maintain a smile during that for the sake of a photograph.
Moreover, dental hygiene is not a priority in those days, and most people do not want to show off mouth full of missing or damaged teeth.
Photos were also expensive, meaning the average person only posed for one or two photos for the rest of their lives.