An example of a difficult riddle is, "When is 99 more than 100?" The answer is, "On a microwave," because when 99 is entered on a microwave, the microwave runs for one minute and 39 seconds, but when 100 is entered, it only runs for one minute.
Another difficult riddle is, "Alone I am 24th, with a friend I am 20. Another friend and I am unclean. What am I?" The answer is the letter X. X is the 24th letter in the alphabet. XX is the Roman numeral for 20. XXX is a label for inappropriate, or "unclean" movies.
Another riddle goes, "I have keys but no locks. I have a space but no room. You can enter, but can’t go outside. What am I?" The answer is a keyboard. There is a space key as well as an enter key on a keyboard.
"What comes once in a minute, twice in a moment, but never in a thousand years?" is another difficult riddle. The answer is the letter M. M shows up once in the word "minute" and twice in the word "moment," but does not show up at all in the words "a thousand years."
Another difficult riddle is, "Which word in the dictionary is spelled incorrectly?" The answer is the word "incorrectly."