One impossible thing to do in zero gravity is to stand up.

Here's why:

* Gravity is what defines "up" and "down." In zero gravity, there is no up or down, just different directions.

* Standing requires resisting gravity's pull. Without gravity, there's nothing to resist, so your legs wouldn't be able to support you in a standing position.

You can still move around in zero gravity, but you would be floating rather than standing.