# raea physics > Make the bakes and play: rigid bodies, soft goo, and parts cut out of a .glb. A `` or `` in a composition plays a bake: the simulation run once ahead of time, so every render of it is the same. A render makes the bakes it needs on its own; `raea physics` makes one by hand from its request, in the composition's asset store. `part` cuts named nodes out of a `.glb` so each can be its own body. ```sh raea physics bake request.json -o bake.json raea physics goo spec.json raea physics part media/kit.glb Wheel_L Wheel_R ``` ## Usage ```sh raea physics bake REQUEST.json [-o OUT.json] | physics goo SPEC.json | physics part SRC.glb NODE... ``` Headless: it needs nothing running and opens no window. Its arguments go to a parser of its own, whose forms are listed above. ## Flags | Flag | Takes | What it does | | --- | --- | --- | | `-o` | `OUT.json` | Also write the result to this file, in place of printing it. | ## See also - [3D](../../guides/3d.md) shows physics in a scene.