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:

Two trains are on the same railroad track; one is headed east the other west, they both leave the station at 1:00 and are travelling at the same speed. Yet they didn't crash into each other. How can this be?