When Reed Richards finished his experimental starship and wanted to take it on an unsanctioned test flight, his college buddy (and former USAF pilot) Ben Grimm agreed to fly it. Together with Richards' girlfriend Sue Storm and her brother Johnny, Ben and Reed launched into space, where they were exposed to unprecedented levels of cosmic radiation.
The radiation triggered mutagenic changes in each of the four passengers. In Grimm's case, he became a brutish creature with orange, rocky skin. Sue's description of him as a "thing" stuck as his heroic nickname, and Thing became a founding member of the Fantastic Four.