Algodoo

Build anything, but with ONLY these bricks.
Do's:Change brick color (except transparent parts) textures/text can be used only for signs, not for a rock texture or something...
Don'ts: change material, tracers, lasers, thrusters (unless you are building a vehicle) change weight, gluing object to background or eachother, springs rope/chains, cut, gears and fixate.

Still thinking about when the contest ends. maybe unexpectedly.
Good luck