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So you've got a container, but nothing to put in it? This tut will show you how to fill objects with “container shaped” liquid.
Extrude Region > Y. That's it, the cup's full! Because you Extracted first, the liquid is a separate object, which can be asigned its own texture map.
Now it gets a little tougher. Now we're going to fill the bowl. Select the bowl and Duplicate it, but don't move it. Leave it selected, and from the Select menu, choose Hide Unselected to hide the real bowl.
Select the edge loops around the inside and outside of the bowl. I like to use the second loop from the top, because otherwise the bowl's too full ;)
Choose Loop Cut, and you should end up with this. Wings has automatically created a new face for the surface of the liquid! Delete the extra peice (the red selected part)
That's it! Show > All and everything should fit right! Don't smooth the liquids. I'd suggest setting the edges of the liquid hard, that makes them render better.