Make no mis-steak
You are on a desert island with a barbecue grill, a raw steak, matches, and a full seven-gallon jug of fuel.
The steak should be slow-cooked for exactly four hours in order to achieve maximum barbecue flavor without overcooking.
Fortunately, you know that each gallon of fuel burns for exactly one hour, and you have two empty five-gallon jugs.
How do you measure exactly four gallons of fuel?
(asked previously here in a significantly more verbose story)