If i could do something with terrains, to be able to shape the terrain and make it fit i n the cup, that would be possible... But i don't think that's possible that way. Have you guys ever made like a glass cup with water in it? is it that hard? I guess you would need a modeling program to shape the water block into the shape of the cup. Any ideas on this?

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I modeled it in Hexagon by extracting the inside shape of the glass and reducing its size slightly.

You dont need hexagon or any other external programs for simple forms. For example i make a drinking glass filled with water the following way in vue:

Make a Cylinder with the desired dimensions of your glass. Copy the object and paste. Make it a bit smaller (downscale, but make sure its centered on the first object, so the smaller cylinder is "inside" the first one.
Now pull the second cylinder a bit to the top that it comes out of the first one a bit.
Mark first cylinder and while holding CTRL select the second. Still hold CTRL, now additionally push "SHIFT" and "D". You have made a boolean difference, resulting an empty drinking glass.

Give the glass a material like standard glass or something else that suits your needs.

For the liquid open your boolean difference and select the second cylinder (not the whole group, just second cylinder). Copy it and resize it a bit vertically downwards, now load a material like "red wine", anything from liquids should work. I like red wine as you can see it quite good, for modelling its good until you are finished. You can scale it vertically to make the glass "fuller" or "emptier".