Rendering liquid in a glass

I am working on a game which will allow the movement of a glass receptacle with a liquid inside. The amount of liquid inside will vary, as will the size/shape of the glass, but I want that liquid to react appropriately to the tilting of the glass. Ie. it will flow up the sides as you tilt it and if you tilt too far it will spill out.

What is the best way to model this? I have looked into Metaballs and they would do the job certainly, but they are probably too much for my needs and, with multiple glasses on the go at different times, are probably too expensive.

I am happy with a flat plane top, but just need the liquid mesh to shape around the interior shape of the glass. I'm pretty sure I'm not explaining this the best but hope someone can point me in the right direction for a solution.