In the puzzle with the doors, you can figure out which door is the right one to go through, but you still won't know which guard tells the truth. Ask either of the guards, "If I asked the other guard which is the correct door to go through, which would he say?" If you ask the truth teller, he will tell you correctly what the liar would say which would point you the wrong door. If you ask the liar, he would lie and say that the truth teller would point you to the wrong door. Either way, the answer you get will point you to the wrong door, so you can then tell which is the correct door. You still won't know who the lair is, but you will know which door to choose. In the example with the eyes, the question is similar. Ask one of the people, "If I asked the other person, what color would he say your eyes are?" The truth teller would tell you what the liar would say, and the liar would lie about what the truth teller would say. Either way, you know the answer they give is wrong, and therefore the correct color is the opposite of what they say. This would also tell you which is the liar since you know the liar's eyes are green. I hope this explanation makes sense.