ITT: Games which simulate physics.
As in, their primary function is to simulate physics.
The two main, most accessible ones I know of are The Powder Toy and OECake. The former has very good heat and electronic models, as well as extreme precision down to the pixel, but has no method of making solid objects that act under physics. OECake is the opposite, there's effectively no measure of precision and material combinations are guesswork, but it can make solid objects that act under physics. Both are 2D.
What others are there?