Parking valet Steven King knows exactly why FlightCar has the most crowded lot at the airport:
"We pay them to park their car," he said.
You drop off your car, they clean it and then hand you a check. There is a catch, course; you have to be willing to let a complete stranger drive your car while you're gone.
"With how expensive parking is these days, I think this is going to be huge," customer Ryan Cave said.
Business has been huge for FlightCar this holiday weekend -- their lot near San Francisco International Airport has been double stacked with cars, and the valets parking them have barely had a second to breathe.
First in San Francisco, now also in Boston, they make money by renting out ordinary people's cars for half the price of Hertz or Avis.
"I might have a harder time letting someone else drive my car, but I definitely don't mind driving somebody else's car," customer Sharmeen Browarek said.
Renters get to pick what they drive.
"They gravitate towards Priuses and Hondas because they save a lot of fuel," FlightCar manager Ric Macapinlac said.
But FlightCar's job is to make every car rentable, even the messy ones.
"They think, 'They're not going to rent my car, you know, it's too dirty,' and then they come back, their car's all clean and they have a $30-$40 check," King said.
You get paid based on how new your car is, and how much it's driven. Even if nobody rents it, the parking and the car wash are still free.