The goal
A single steel needle is buried somewhere in the stack. Pull the hay apart until you see it glint, then grab it.
Making money
Coins, keys, rings and other junk are hidden in the hay too. Pick them up, fill your backpack, and sell them at the Trading Post to buy better tools.
Detector
The metal detector beeps faster as you get closer to metal. From Mk III it can tell you when it's the needle.