The planet Pfooey is inhabited by robots. Half of the robots always tell the truth, but the other half have been affected by a bug in their software that causes them to lie at all times. All of the robots look exactly alike, so it can be confusing to figure out which robots are truth-tellers and which are liars.
One day, a visitor to Pfooey stopped at a fork in the road. Two robots approached the tourist from the road on the left. The tourist said, "I'm on my way to Bitborough. Which road should I take?"
One of the robots replied, "We've just come from Iteropolis. If you asked me, I'd say Bitborough is to the right." The other robot pointed to the first and said, "That one's a liar. Bitborough is to the left." The tourist thanked the robots, then continued on the road to the left until she reached her destination a short time later.
How did the tourist figure out which way to turn?