How do you find a needle in a haystack?

Use a metal detector and hope for the best, or burn the haystack.

Improved

Meaning: extremely difficult; extremely rare

Example:

Trying to find your class room in the engineering building is like trying to find a needle in a haystack.

I can only think of three options:

1. Using a magnet to attract the needle,

2. Burning the haystack so only the needle will be left behind, and

3. Put the whole lot in a basin of water. The haystack will float while the needle will sink to the bottom of the container.