A man goes into a pet shop to buy a parrot. The shop owner points to
three identical looking parrots on a perch and says:  "The parrot on the
left costs 500 dollars".

"Why does the parrot cost so much?" asks the first man. The owner says
"Well, the parrot knows how to use a computer".

The man then asks about the next parrot to be told that this one costs
1,000 dollars because it can do everything the other parrot can do plus
it knows how to use the UNIX operating system.  Naturally, the
increasingly startled man asks about the third parrot, to be told that
it costs 2,000 dollars.  Needless to say this brings the question "What
can it do?"

To which the owner replies "To be honest I have never seen it do a
thing, but the other two call him boss!".