i've got one!
you're walking to the city, and you come to a fork in the road. there is no signage so you don't know which path to take to get to the city. you aren't carrying anything except a water bottle...so no map. at this fork there are two houses, in which two familes live. one family always tells the truth, and one family always lies, but you don't know which family lives in which house.
what question would you ask to each family to find out the way to the city? you can only ask each family one question, it has to be the same question that you asked to the other family.