Link on your phone and don't be shy, walk head first against that ball by touching the screen!

Exercise : make a 3D fountain of balls, ideally one that makes music (examples on youtube are legion). Note that physics are still a bit imprecise so like in the demo display the wireframe of all objects to make sure they behave ... mostly properly.

Extra exercise : make the table of your room solid (you can cheat and make it a large box) then push it around.

Done? Go back to learn more.