I noticed that puzzling stack exchange didn't have my favorite riddle. It isn't too uncommon so someone else might have heard it (and a quick google search can easily find the answer) but someone out there might enjoy it as much as I do. So here it is:
In a marble hall white as milk,
lined with skin as soft as silk.
Within a fountain crystal-clear,
a golden apple doth appear.
No doors there are to this stronghold,
yet thieves break in to steal its gold.
What is it?
"thieves" is a strong word. If a person owns the marble hall then they also own the gold.