Riddle: With marble walls as white as milk, an inner skin as soft as silk, inside a fountain crystal clear, a golden apple does appear. There are no doors to this stronghold, yet thieves break in and steal the gold. What is it?
Answer: An egg. The marble wall is the outside of it, the fountain crystal clear are the whites, and the apple is the yolk! Obviously, no doors to an egg, but the 'thieves' break into it by breaking it and getting the yolk (stealing the gold)!