You are camped in a wooded area with a group of field biologists. A mile or so away is an area in which dwell two species of Bufo toads—the American toad and Fowler’s toad. Similar in looks, both species have mutated so that contemporary populations can talk to humans. To test the suspicion that pollutants are responsible for this amazing evolutionary development, you are sent out with a rucksack full of sterile vials to sample the pond water near the toads. You have been warned that the toads are tricky. They've developed a wily and vindictive process of speech: in the presence of a human, an American toad always tells the truth; a Fowler’s toad always lies.
Having collected your samples, You head back to camp, burdened by the weight of the water samples. Shadows fall like veils upon the woods and soon you are lost. You reach a fork in the path. On the ground before you squats a toad, half obscured in the twilight.
“I beg your pardon, kind toad,” you exhale, “I'm lost. Will you help me choose which of these two roads leads to my camp."
The wily toad offers help in this form. It will answer one and only one question. But zounds! Is this a truth telling toad or a liar? You don't know the difference between an American toad and a Fowler’s toad!
Think up ONE QUESTION to ask this toad so that you will be certain which path to take. (Assume it knows which is correct.) Act fast. If you figure out what to do soon enough, you will make it to camp in time for supper.
Solution
There are several slick possibilities. The trick is to get the toad to tell you what he’d do. For example, you might ask, “If I were to ask you which road leads back to camp, which would you point to?” In response to this, the Fowler’s toad has to lie and point the opposite way it would actually tell you (opposite the wrong way, i.e. the right way). Either species’ answer will reveal the correct road to take.