If a fly lands on you while youre making a silly face then your face will freeze that way forever.
Demi remembers hearing this from her mom at a young age and even her older sister would tell it to her. Even though she does not seriously believe the saying she still tells it to people when they intentionally make strange faces. She has a few classmates who I have heard the saying before and almost everyone in her extended family is also familiar with it.
This is an example of a cautionary saying to prevent children from doing something that adults do not like. It seems like an arbitrary combination of an event that could possibly happen (a fly landing on someone) but the chances of it happening are slim (while they are making a funny face). The sole purpose of this phrase is to scare children into not making strange faces although it possibly has an opposite effect and makes them more curious.