Give them a bill

A doctor and a 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 stop people from asking you for legal advice when you're out of the office?"
"I give it to them," replies the lawyer, "and then I send them a bill."
The doctor is shocked, but agrees to give it a try.
The next day, still feeling slightly guilty, the doctor prepares the bills.
When he goes to place them in his mailbox, he finds a bill from the lawyer.