The zero-gravity environment inside the International Space Station gives astronauts ample opportunity to have a little fun--as seen in this new space bubbles video in which NASA astronaut Don Pettit builds some bubbles.

Pettit isn't content with ordinary bubbles familiar to Earth-based bubble blowers. Using a hypodermic syringe, he creates bubbles inside a sphere of water--bubbles inside bubbles.

The sphere of water Pettit toys with in the video is actually an antibubble—a droplet of liquid surrounded by a thin film of air.

"So that’s like a thick spherical shell of water," he says. Then he drops a bubble inside. "Oh wow, look at that."

The demonstration was recorded this month as part of Pettit's physics-in-space video series Science off the Sphere.

In the video, Pettit points out that the bubble floats to the center of the sphere and then asks: When the water sphere rotates, why do the bubbles center themselves?

Know the answer? Tell us in the comments.