Just a heads up: Proof is a measure of alcohol content per volume.

If you pour 1 cup of 100% alcohol into 1 cup of water, you have ~1.5 cups of ~67% alcohol (by volume). This means that the proof of the final drink is roughly 33% more than intuition might tell you.

There is no law of conservation of volume. The alcohol molecules are much bigger than water molecules. Water molecules nestle into "holes" in between the alcohol molecules.