I found this on Reddit and it’s pretty crazy. Phun is Master of Science Theses by Computing Science student Emil Ernerfeld. He calls it a

A fun 2D physics sandbox

But it’s really hard to describe. Checkout screenshots and the especially the video on the media page.

I could see this being a neat education tool to explore gravity, motion, etc… I’ll certainly be installing this on the kids computer (Linux download available!) so they can tinker! I originally suspected he used Flash but not according to the site:

Phun was coded in C++ using OpenGL, GLEW, SDL (for window management), SDL_image (for reading images) and boost, including boost_filesystem. Everything was coded by me, including the physics engine and user interface.

Very cool!