When you have an empty cylindrical cup the centre of mass is about halfway up, the same is true with a full cup, but partly full and the combined centre of mass of cup and liquid is below hafway.
I found out that for a prism being filled up lengthwise with liquid, the total centre of mass is lowest when the height of the liquid is the same as the combined centre of mass.
Is this true for any shaped vessel for example a conical cup or a curved cup?