from Howstuffworks

On August 10, 2006, authorities in Great Britain announced that they had arrested several people in connection with a plot to attack airplanes with liquid explosives. The attackers planned to disguise the explosives as ordinary liquids and smuggle them aboard. For this reason, authorities in both the United States and Great Britain warned all passengers that liquids would not be allowed in carry-on luggage until the crisis passed. The ban on liquids included hair spray, shampoo and beverages — items people travel with all the time.

So what exactly are liquid explosives, and what do they do? Most explosives work in basically the same way. They burn or decompose very quickly, producing lots of heat and gas, which rapidly expands and can tear things apart. Usually, an explosive material requires some kind of stimulus, like heat or shock energy, to get the process started. You can learn more about different types of explosives in How Bombs Work.

Many people don’t think of explosives as liquids, though. Most of us imagine them as solids, like gunpowder or C-4. But you’ve probably heard of one liquid explosive already — nitroglycerin. Nitroglycerin was invented in 1847, and it’s made by adding acids to glycerin. Since exposure to it dilates people’s blood vessels, it’s used as a medical treatment for angina pectoris, or heart pain.