One of Seattle’s most beloved ongoing promos, Free Fry Friday at Pike Street Fish Fry, happens tomorrow.

The drill you know: free fries, no purchase necessary, 3pm to 7pm, every first Friday of the month. What you probably don’t know are the numbers that make it ongoing.  

It’s what’s known in the biz as a loss-leader strategy: Offering something for free that’s cheap anyway, like fried potatoes, to get folks in the door to pay for something that can be marked up, like beer. Overall, on Free Fry Fridays the tiny fry joint sees about a 20 percent increase in happy hour patrons, who then stay long enough to open their wallets. “Typically people come out and end up spending extra time at the bar when they realize fries are free,” says marketing manager Guy Keltner. Beer sales increase by some 15 percent on FFFs, with Hamms the top seller owing to its buck-a-can price tag from 3pm to 4pm. Can’t beat that for day-drinking. (Unless it’s with tequila, the top spirit PSFF sells that day.)

Of course it's the crispy, double-blanched fries that really fly on these days, when PSFF fryers churn out some 100 extra orders. Sales of fish tacos—which aren't free—double during FFF.