Watch NASA's Scott Kelly play ping pong with a water droplet in space

This is the longest International Space Station mission in history. And it looks like they're having fun.

"Neat stuff," astronaut Scott Kelly said.

See the action below in the gallery:

That's Scott Kelly playing pingpong. He's been on the ISS for 10 months.

He uses hydrophobic paddles. They work "kind of like a rain coat."

The ball is a water droplet in microgravity.

See more of Scott Kelly's space mission below:

So, what else are they doing?

"Our mission here is to understand the effects, mostly the effects on our physiology and psychology, of long duration space flight, so we can eventually go onto Mars," Kelly said.

The one-year mission ends in March.

Watch more with Scott Kelly in this exclusive interview below:

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