If you've popped a Champagne cork, you've probably noticed the cloud of white that flows out. But if you're really observant — and the conditions are just right — you might see the cloud turn blue.
When the cork is popped, the CO2 inside the bottle expands, and the temperature drops. When the bottle is cold, the drop can form a thick, white cloud.
But at warmer temperatures, there's less of a cloud. Instead, CO2 and water vapor in the bottle are at the perfect density to form blue-tinted ice crystals, creating a thin blue fog.
But when the CO2 runs out, the fog becomes white-gray. It happens in a fraction of a second, so if you're trying this trick at your next party, make sure everyone is watching closely. And maybe have them wear eye protection.