system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did "cheese" become the word to get people to smile? When did people start smiling in photos?
gpt
To answer the second part of your question - people started smiling (on a regular basis) in photos when taking a photo stopped requiring you stay perfectly still for half an hour. Or 10 minutes. Or 5 minutes. Or 1 minute. It's a lot harder to hold a smile for a good length of time than it is to just look very passive - and in the early days of photography you needed to stay perfectly still for a long time. That's why a lot of people have a misconception that people back then were miserable. 

Nonetheless, you *can* find old photos of people smiling. It's actually delightful to see.