Cabbies have always had fertile imaginations, but what about a guy who invents a surcharge out of thin air? Don't scoff. It happened the other day to Mary Brew of Northwest.

She hopped a cab on Capitol Hill and asked to go to 19th and M streets NW. As the cab pulled to the curb, she asked, "Three dollars and ten cents, right?"

"No, $3.85," said the cabbie. "Air conditioning fee."

Mary shot him a quizzical look, whereupon the guy said, "Hey, the air conditioner's not free, you know." He said it "so forcefully that I figured he must know what he's talking about," Mary told me later. So she paid the $3.85.

In case you haven't guessed, there's no such thing in the D.C. taxi fare schedule as an air conditioning fee. If a cabbie tries to soak you for one, tell him that what he ought to go soak is his head.

Happy ending for Mary, by the way. Still suspicious as she left the cab, she wrote down its number. Then she checked with the D.C. Public Service Commission, which informed her that she'd just bought one 75-cent share of stock in the Brooklyn Bridge.

But because Mary had the cab's number, hack inspectors easily located the driver. As a result, Mary will soon have the delicious experience of watching him reach into his pocket, find three quarters and fork 'em over.