Some billionaires spend their spare time and money building ever-bigger yachts.

Tesla Motors CEO Elon Musk's passion is for rockets, and last week, he took to Twitter to argue with Amazon CEO Jeff Bezos over whose rocket was, well, not bigger but better.

Bezos' rocket startup, Blue Origin, said that it had fired a rocket to the fringe of space, 62 miles above Earth. Then it returned, landing less than 5 feet from the center of the platform from whence it had launched.

It was a breakthrough in space travel, in which rockets traditionally have been discarded after every flight, and a goal that Musk's company, SpaceX, also has pursued. In a Twitter post, Bezos called his lightly used rocket "the rarest of beasts."

"Not quite rarest," Musk retorted, noting that SpaceX started suborbital flights in 2013 with vertical takeoff and landing and now is trying to reuse boosters from orbital flights -- but hasn't yet. Later in the day, a CNN reporter asked Bezos whether Musk was correct.

"No, he's not," Bezos replied.