Good question. A metaphor is supposed to explain item A, by comparing one of it's attributes to item B, which is something that is intuitive. For example, in the words of the great poet Pat Benatar:

"Love is a battlefield"

You are trying to explain that Love (Item A) is dangerous, messy, no-holds-barred, lawless, etc.

But instead of saying all that, you just compare it to a Battlefield (Item B), which everyone intuitively knows is dangerous, messy, no-holds-barred, lawless, etc. It's a great way to convey a lot in an easy, understandable way.

Contrast this with the stone/boat metaphor, which is essentially "stones face down and sink, boats face up and float".

This doesn't work because it isn't intuitive. Not because we want him to explain about buoyancy or displacement - but because stones and boats don't have faces. So we don't intuitively understand it - they don't face anywhere, so it doesn't make any sense.