Goldfish
For his birthday, Little Johnny's parents gave him a bowl of goldfish. His mother carefully explains to him how much responsibility comes with owning a pet and tells him to make sure he remembers to give the fish the care they need every day.
Johnny is responsible with the fish for about one week, after which he begins forgetting to look after them, leaving his parents to feed the fish and clean the tank. After a couple of days, Johnny's mum and dad bring the issue up to him.
"Now Johnny," says his mother. "You have to be more responsible."
"Do you know how many times those fish would have died if we hadn't been there?" his father adds.
"Uhm, I don't know," Johnny replies. "My guess would be Once?"