I believed that Pennywise from Stephen King's IT lived in the bathtub so I would always turn on the hot water before unstopping the drain in order to scald him.
A childhood belief about bathrooms

others believed things like this
- I used to think that trolls lived in my shower drain and showered with my dirty water— Troll bather
- how many us believed you would go down the drain with the bath water? I did!— Amy
- I used to believe that if you kiss a frog then the would would really turn into a king.— jing
- I used to believe that if i splashed water on the wall during a bath, the water would turn into spiders— justin
- ***I used to believe that water towers was a place that someone lived and they controlled the water.— C H R I S T E N
- I used to believe that if I ate too many watermelons I would turn into water...— Peter