From [wu : riddles]

An enemy submarine is somewhere on the integer number line. It is moving at some rate (again, integral units per minute). You know neither its position nor its velocity.

You can launch a torpedo once each minute at any integer on the number line. If the the submarine is there, you hit it and it sinks. You have all the time and torpedoes you want.

You must sink this enemy sub - devise a strategy that is guaranteed to eventually hit the enemy sub.