This is a semi-fileld helium baloon , Press the right mouse button and select "put atom here" then go in he ACTIONS/ATOMO and put -100 as a mass (this will make so that the atoms you put in the world have a negative mass and therefore go up). Now you can slowly fill your baloon .. slowly = not more than a atom a second or it will blow really bad ..
One interesting thing to do is to lower the fluidfriction to values like 990 or so.. you will notti ce that the filling becomes very realistics but when it blows up it is not much :-)
