As a fan of The Incredible Machine and Crayon Physics: holy crap, that’s awesome! The art and the level design is top notch, the sounds are satisfying, really the only tiny UX issue I had is the one someone else already mentioned (maybe pressing restart should automatically pause too). You did an excellent job especially considering the 3-hour timeframe, congratulations.
I’m stuck on level 6 for the moment, but I’ll give it another go after a coffee :)