model.addTest(new ConveyorBelt());
model.addTest(new SphereStack());
model.addTest(new Tumbler());
model.addTest(new PistonTest());
model.addTest(new PyramidTest());
model.addTest(new CircleStress());
model.addTest(new DominoTower());
// more interactive..
model.addCategory("Interactive");
model.addTest(new VerticalStack());