Principia is a free and open source physics-based sandbox game. In Principia, you can build contraptions and simulate them in the physics simulation. This could be a mechanical contraption, an RC car, or a pinball game. Principia also contains a LuaScript object which allows you to write and create Lua programs that can interact with and affect the simulation.

Principia also features the Adventure game mode. In this game mode, you take control of a small robot which can do whatever you decide it to do. It can take part in a giant battle with enemy robots, or it can take control of the giant robot and wreak havoc on the world.

Beginning its life in 2013 as a paid game for Android and later Windows, it has now become an open source project. Be sure to check out the source code repository on GitHub and the Principia community site principia-web where user-made levels are uploaded for others to play and enjoy.