Next time you want to trip someone up in hangman, try using the word "jazz." Mathematician Jon McLoone used a computer simulation that helped him uncover that "jazz" is the hardest word to guess in the game.
The reason for this is simple: shorter words give the guesser fewer chances to get the right letter, and vowels are the most commonly guessed letters. The least guessed letters are those that are used less frequently in the English language, such as j, z, q, and v. Given that 2/3 of the letters in jazz are on that list, the word ends up being very hard to guess.
Of course, if your opponent is a Flosser, they might be on to you now!
Image courtesy of Great Beyond's Flickr stream.