This is probably one of the most fun simulation/games that I have played. It’s a physics sandbox game very much like Powder Game. You can build a variety of different things, and Powder Toy will accurately simulate pressure, heat, wind, and gravity.
I’ve spent countless hours building things from nuclear reactors powering cities to bunkers being used to test nuclear weapons. I’ve even built a fusion reactor (sort of…it’s not actually possible AFK). Best of all, Powder Toy is free! Design, Build, Blow up. :)