Perhaps most surprising is that this same principle also explains how many high explosives work. They are called "high" because their explosive reaction expands through a supersonic pressure wave that travels much faster than ordinary burning, making them far more powerful than low explosives like gunpowder. Each successive bit of material in a high explosive ignites when the pressure wave compresses and heats trapped microscopic bubbles of gas. When manufactured without bubbles, even extremely powerful high explosives can be impossible to detonate. Without gas to compress, there is no way for the detonation wave to heat up neighboring areas.