Squigly Squigly's Brain Teasers

Question:

In marble walls as white as milk,
Lined with skin as soft as silk,
In a fountain crystal clear,
A golden treasure does appear.
There are no doors to this stronghold,
Yet thieves break in and steal the gold.

What is it?

Answer:

An egg.

Try another Brain Teaser:

Jack tells Jill, "This isn't the $5 bill you left on the the table. I found it between pages 15 and 16 of Harry Potter."

Jill retorts, "You're lying and I can prove it." How did Jill know?