Mixing Coke & Milk

What we did:

1. We opened a fresh, new bottle of coke and poured some of it out.
2. We carefully poured 3% milk into the bottle with the coke until the bottle was completely full.
3. We replaced the bottle cap and screwed it on tightly.
4. We then needed to exercise some patience. We observed the milk and coke mixture every half-hour.
5. Slowly, but surely, the milk settled at the bottom and the coke became invisible.

Why did this happen?

The separation happened because of a reaction between phosphoric acid in the coke and milk. Phosphoric acid molecules attach to the molecules of milk, increasing the density and separating them from the rest of the liquid. The remaining liquids, having less density than the phosphoric acid and milk molecules, floated on top.