There was a man driving around in his Mercedes through the countryside, and spots a field of grazing sheep. He stops his car and walks up to the shepherd and says, "I'll make a deal with you. I will guess how many sheep you've got in this field, and if I guess right, I get to take one of the sheep home with me. If I guess wrong, you get to keep my car."
The shepherd thinks for a moment and finally agrees.
"137," says the driver.
"You're right!" says the shepherd, and dutifully hands over the sheep.
The man begins to walk away, and the shepherd stops him. "Now I've got a deal for you. If I can guess what you do for a living, I get to keep your car. If I'm wrong, you can have all my sheep."
"That's a deal," says the driver.
"You're a consultant," says the shepherd.
The man was dumbfounded as he handed over his keys. "How did you know?"
"Easy," says the shepherd. "You come here uninvited, you tell me something I already know, then you charge me for it."