what is the best way to model a liquid in a cup? like a glass so caustics can be used?
Model the cup, then put a fluid sim object inside the glass.
If you don’t plan on making an animation (liquid motion) just duplicate the verts of the inside of the cup, seperate those, then add faces for the top of the liquid. You might need to scale the liquid down (just a very little bit!) so the cup and liquid faces aren’t directly on top of each other.