Phun is a very good 2D physics sandbox game.
To get started i recommend to first “load a scene” from the given examples and then play around with it. By doing so you will understand how things work and what is possible in the game. You can also download creations from other players or upload your own.
There are also many youtube videos floating around where people showcase their complex and interesting creations in motion.
Info: The game can get a bit slow when you make lots of cpu heavy stuff like liquifying huge objects into small water particles.
Looks very nice!