When you do something you usually only do alone, like sing while washing the dishes or making food for example, and someone was standing at a distance in the same room and you notice, so you ask them how long they've been there.
guy 1: *singing*
guy 2: hi
guy 1: how long have you been standing there?
When water is completely still and likely doesn't move and likely contains amoeba or dangerous bacteria
example: exploring an abandoned building is fun untill....
Still water ðððð
Those who know : ðððððð