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 {| ! needle !! hay
 * metal || vegetable
 * silver-colored || tan-colored
 * not flammable || flammable
 * magnetic || nonmagnetic
 * denser than water || floats on water
 * sharp || dull
 * rigid || flexible
 * etc. || etc.
 * }
 * denser than water || floats on water
 * sharp || dull
 * rigid || flexible
 * etc. || etc.
 * }
 * etc. || etc.
 * }
 * }

This listing reveals multiple possibilities that can be considered with the question of hint #1, to select the optimum strategy. A pitfall, however, is the possibility that the needle may be stuck into a straw and thereby exhibit straw-like properties.

Answers (not in order):
 * Burn down the haystack, then sift through the ashes.
 * Buy an airport X-ray machine and pass all hay through it.
 * Throw away the haystack and buy another (hay is cheaper than time).
 * Go get another needle.

Problem: Search for the Top Quark.
High-energy physicists, needing to test theoretical predictions concerning subatomic processes, attempted to detect a subatomic particle that they called the top quark. They wrote computer programs to scan through a very large number of potential events and discard most of them. The remaining events then could be examined more carefully, to see if they might be caused by the top quark. Effectively, hay was plentiful, and the needle was only hypothesized to exist.

A major concern in designing the computer program was the relative impact on the experiment of two types of errors. An alpha error is a false positive (calling a straw a needle). A beta error is a false negative (missing a needle).

As I write this, the top quark finally has been detected.

Tips on Experimental Design and Execution
“The general who understands the advantages of varying his tactics really knows the art of war.

“The general who does not appreciate the need to vary his tactics cannot turn natural advantages to account. ..

“The wise man considers both favourable and unfavourable factors, the former to pursue his objectives and the latter to extricate himself from difficulties.” [Sun Tzu, ~500 B.C.]