Video: Molten salt is, as its name suggests, salt in liquid form. It doesn’t get there until between 260 and 600C, making it one of the worst things you can drop into a glass vessel full of cold water.
It’s not just great at making fish tanks explode though: molten salt can transfer heat, conduct electricity, act as a solvent and can be used in some fuel cells. Maybe TheBackyardScientist will get around to talking about all that in his next video, right after he finishes picking glass shards out of every part of his lawn.