Once again, TouchArcade proves to be a useful resource for iPhone and iPod Touch fanatics.
AquaForest is a cool little app that lets you play with physics. Interesting 2D physics playground, where you can create and destroy solids, gasses, fluids, heat and cold sources, and control gravity just by tilting the screen.
This app very much reminds me of a desktop physics app called Phun. This totally appeals to my Lego sensibilities. You get to carefully construct interesting structures of varying complexity. Then have a lot more fun blowing them up.