[quote author=Wieslaw link=topic=722.msg4572#msg4572 date=1249669191]
There is something wrong with the model, not just with the applet. As you can easily see the distance between the particles on the surface changes. Therefore some of the particles of the surface must go inside. I guess that the theory does not include that particles have volume.  It treats them as they were points. Can anybody solve the puzzle?

Yes. It is just a simple model. The particles are treated as points and it did not consider the conservation of the whole volume (it is conserved on average).