<mujoco>
  <worldbody>
        <light diffuse=".5 .5 .5" pos="0 0 3" dir="0 0 -1"/>
    <geom type="plane" size="10 1 0.1" rgba=".9 0 0 1"/>
    <body pos="0 0 1">
      <joint type="free"/>
      <geom type="sphere" size=".1" rgba="0 .9 0 1"/>
    </body>
  </worldbody>
</mujoco>