A man goes into a pet shop to buy a parrot.
The shop owner points to three identically looking parrots on a perch and says:
"The parrot on the left costs $500."
"Why does the parrot cost so much?", asks the guest.
The owner says,
"Well, the parrot knows how to use a computer."
The man then asks about the next parrot. He is told that this one costs $1000, 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. He is told that it costs $2000. Needless to say this begs 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!"