Q. A traveler meets a native in the jungle at a the fork in the trail, where only one path goes to the village. The traveler does not know which path to take and knows that the native could be either from the tribe of Truth-tellers, who always tell the truth, or from the Liars, who always lie. What is one question he could ask the native to discover the path to the village?

Ans. There are at least a hundred variations on this problem and they are not all based on the same idea. This is one of the simplest but it is still tricky. The idea is to find a question such that either native will answer the same. That requires that the question include some sort of double negative, which will cancel out the fact that he may be lying. One example is "If you were from the other tribe, which path would you say goes to the village?" The truth teller will point to the wrong path, and so will the liar (think about it). So whatever path is pointed to, take the other one.