Susan Forrester, age 15, of Niagara Falls, Ontario, Canada, for her question:

HOW IS NITROGLYCERIN USED?

Nitroglycerin is an oily, explosive liquid that is made by treating glycerin with nitric and sulfuric acids. The wonderful thing about this mixture is that it can be used both in medicine as a heart stimulant and as the principal explosive ingredient of dynamite.

Certain types of heart disease and blood circulation problems are treated when the doctor prescribes nitroglycerin tablets. It's a powerful but very effective medicine.

When used as an explosive, nitroglycerin expands to form gases that take up more than 3,000 times as much space as the liquid. Pow!

An Italian chemist named Ascanio Sobrero discovered the explosive in 1846. In 1864 a Swedish chemist named Alfred Nobel perfected a detonating cap for nitroglycerin and three years later he invented dynamite, a safer and more convenient explosive.