For a limited time, fans of Burger King's Whopper can buy it for a penny — if they're at a McDonald's restaurant.

Burger King is promoting the relaunch of its app by trolling its rival.

The app uses geofencing to determine if the customer qualifies for the promotion. If you're within 600 feet of a McDonald's location, the burger is yours for a fraction of its usual price. Once the order is placed, the app will lead customers to the closest Burger King. The promotion begins Tuesday and will end Dec. 12.

The enticement of a one-penny Whopper should encourage customers to download the company's app, calling attention to how restaurants are using their apps to interact with their patrons.