A doctor and lawyer are talking at a party. Their conversation is constantly interrupted by people describing their ailments and asking the doctor for free medical advice. After an hour of this, the exasperated doctor asks the lawyer, "What do you do to

 The lawyer replies, "Oh, it's quite simple. I send them a bill for my advice."

The doctor chuckles and says, "That's a good idea. I should try that."

Later in the evening, another partygoer approaches the doctor with a medical question. The doctor smiles and says, "Of course, I'd be happy to help. But first, let me send you my bill."