hi ,i 've got an iPhone and iPad and often heard about water damage check mark. People want to know if their Apple products are waterdamaged or not. I am wondering if something gets damaged, can't it tell from the trying to start the machine and see if things still going alright? For example, the machine doesn't charge or doesn't turn on, or showing abnormal screen which obviously tells it has problems. Why still need to check the water damage mark or any other indication to confirm/test if it's damaged?