Elon Musk has poked fun, in a rather crude way, at Jeff Bezos' new spacecraft.

On Thursday, Bezos' space company Blue Origin unveiled its first lunar lander called "Blue Moon."

Musk, who is the founder of rival space firm SpaceX, tweeted an altered picture of Blue Moon.

Instead of reading "Blue Moon" on the side of the spacecraft, the image in the tweet reads "Blue Balls."