Place weights on the teeter-totter first by clicking on the block you want to place and then by clicking on the on the spot along the teeter-totter where you want to block placed. To test your balance, select from the menu on the top left hand of the applet (under "Choose an Action"). You can create some weights of your own by selecting a different value from the "Create your own Mass" menu. Finally, each time you click on the "Mystery Mass" button, a new weight is assigned to the black block labelled '??'.