I designed this to be released with my version of the Gold Panning system, but it can be used for mining, fishing, etc. Basically, all you have to do is provide a chance of randomly receiving one of these beauties while performing common tasks, and it will continue to spawn duplicates of itself inside the player's pack until they answer the (very easy) question. When they answer it correctly they are awarded a small amount of coins, and all of the boxes are safely deleted. You can of course modify this to suit your economy.
It has been tested extensively, and should not cause any problems. The box only functions inside a player's pack, and is not movable.