What is Mister Fantastic's real name?

Walking around calling yourself "Mister Fantastic" is a pretty bold choice. There's no superhero named Great Guy or Amazing Dude, or Superma... nevermind. The Fantastic Four's resident genius spends most of his time stretching his brain around impossible scientific problems, but Stan Lee and Jack Kirby knew they needed to give a more humble alias for when he wasn't dealing with giant purple space men (not space clouds). What is Mister Fantastic's real name?