Smoking Goblet Potion
In Harry Potter and the Prisoner of Azkaban, Professor Severus Snape prepares a mysterious-looking goblet of smoking potion to cure a mysterious malady suffered by Professor Lupin.
Make your own mysterious smoking potion.
Dry ice is frozen carbon dioxide, which is non-toxic. This extremely cold solid produces carbon dioxide gas as it warms up. When placed in warm water, the cold gas joins with water vapor, making a combined visible vapor that is denser than air. This causes the carbon dioxide to sink as it expands past the goblet's rim.
Place a chunk of dry ice in an empty aquarium or other transparent container. Allow five minutes for a layer of invisible carbon dioxide gas to collect in the bottom of the tank.
Blow bubbles outside the container and watch them sink in the air. Then blow bubbles inside the container. The bubbles float in the tank, since they aren't as dense as the layer of carbon dioxide.