What are some difficult-to-solve riddles?

One of the most difficult riddles is "What has four fingers and a thumb, but is not living?" This riddle is difficult because answerers usually focus on the living part and do not see the relatively simple answer.

Another difficult riddle is "I have keys but no locks. I have a space but no room. You can enter, but can’t go outside. What am I?" This riddle is unique because it is abstract in its presentation but literal in its answer. "I can only live where there is light, but I die if the light shines on me. What am I?" can be difficult to answer as well because of the abstract delivery.