A classic trick that is available again- and now updated to make it EVEN EASIER to do!
Start with your props: resting on a tray are a glass full of milk and a glass full of water, along with an empty milk bottle, and two empty covers. Pour both the milk and the water into the bottle, mixing the liquids together. Cover the empty glasses with the two metal covers. At your command, the liquid mixture in the bottle visibly diminishes, until the bottle is completely empty! When the covers are lifted off the two empty glasses, the milk is back in its original glass, and the water in the other glass!!
You can use any other liquids instead of milk and water, too!
You supply the liquid.