Alice and Bob are imprisoned. Alice can see 12 trees and Bob can see 8 trees. Together, they can see all the trees, but no tree is seen by them both. Neither knows how many trees the other sees, and they can't communicate with each other. Every day, Alice is asked, "Are there 18 or 20 trees?" If she passes, then Bob is asked the same thing. If he passes, the process repeats the next day. If either guesses the total number of trees incorrectly, they're both imprisoned forever. If either guesses correctly, they're set free. They both know these rules. Can they escape by giving an answer with certainty? Or is the best chance they have at escaping a random guess? Watch the videos to get the answer, and explore more challenging puzzles.