Build a working chain-reaction machine from ramps, fans and gears so the ball finds its way home, then rewind the physics to see exactly where it went wrong.
Each level gives you a limited tray of parts and a ball that needs to reach the goal. Place parts on the field, aim them, and hit Run to drop the ball, the deterministic simulation plays out the physics. If it misses, scrub the timeline to rewind and step frame by frame, find the flaw, adjust, and try again. The more parts the ball actually uses, the higher your contraption scores.
Contraption is a physics puzzle about building elaborate chain-reaction machines, training cause-and-effect planning.
A limited tray of parts, a ball that must reach a goal, and a deterministic simulation is the Rube Goldberg machine as an engineering puzzle.
Determinism is the original guarantee: the same layout always produces the same run.
That matters more than it sounds. If the simulation were noisy, a failed attempt would tell you nothing; because it is exact, every run is a measurement, and the puzzle becomes iterative design rather than repeated hope.
Further reading: A level that proves itself solvable
Contraption is an original free browser physics puzzle about building elaborate chain-reaction machines, with a scrubbable deterministic simulation for tuning every bounce. Made in-house for PlayPendium.
The game never hand-authors a puzzle. It grows one from a single number, and then it plays that puzzle against itself to make sure it is neither trivial nor impossible before you ever see it.