Poison

Dad to kids = Do not go on that bit of grass over there. I’ve just put poison on it and it will kill you.
Goes off to do something else. Son comes up a little while later.
Son = Daaadd, why can poison kill you?
Dad = because it has bad things in it.
Son thinks for awhile.
Son = what bad things does it have in it?
Dad = (getting fustrated) oh, I dunno, arsenic.
Son thinks on this a little while.
Son = Dad, who’s Nick???