The Fork in the Road
You reach a fork: one road leads to safety, the other to doom. A guard stands there who is either a knight (always tells the truth) or a knave (always lies), but you don't know which. What is the minimum number of yes/no questions that guarantees you identify the safe road?
Related problems & prerequisites
Worth solving first
Source: Classic knights-and-knaves puzzle of the kind popularised by Raymond Smullyan. Statement written for AxiomIQ.