The same falling crates, written as data instead of code: a scene document, the exact JSON an editor saves, run through parseSceneDoc and sceneFromDoc. The engine stores colliders and reads them back but cannot move them; installPhysics2d() is what makes them fall.
The document is verbose because it is a file format and not an authoring API. In an editor you would never type it: you would drag an object and tick "Collider".