There's a catch, of course. Customers have to buy at least $1 worth of food, for the free medium fry. They can also only use the free fry deal once each week at a participating McDonald's, the website's fine print states.
Free fries aren't the only deal being offered. The site also says customers can get a free medium McCafé with a $1 purchase and $2 off a signature crafted sandwich once a week for most of April.