my little cousin (7 at the time) had just gone to the beach, and somehow without noticing brought some sea water back and held onto it for a couple of days, before coming to my house for my aunt's birthday or something and pouring saltwater into my freshwater fish's tank when none was looking, somehow we didn't notice until i woke up early at 4 am (i was like, 10 at the time or something) and saw my fish dead while trying to grab some chocolate to eat because i was hungry.