When you say "eggplant", especially when you say "zi", the corners of your mouth will naturally rise, making facial muscles close to the shape when you smile. Just like Americans like to say "cheers" when taking pictures, it is the same reason. In addition, when you say eggplant, you will grin and smile more.

Therefore, it should be the most easygoing, beautiful and natural for people to find photos in this state.