It's the letters of the alphabet, from "'H' to 'O'." Hilarity presumably ensues in response to this amusing riddle, causing riots, mass suicides, and gonorrhea the likes of which you haven't seen since you banged that midgit transsexual in Vegas on your honeymoon with your chain-smoking, compulsive-gambling alcoholic wife, whom you had then happily known for four hours.
EDIT: Okay, here's one:
What are the next two letters in the following series and why?
W, A, T, N, T, L, I, T, F, S, _, _