Riddle: A snail is at the bottom of a well that is eighteen feet deep. The snail can crawl up the wall three feet, but when it rests it slides back down two feet. How many tries does it make before it gets out of the well?
Answer: 16. On the fifteenth try, the snail reaches eighteen, but slides down to sixteen feet. On the sixteenth try, it crawls up past the eighteenth foot and is free.