Linda Young, aged 10, of Clarksville, Tenn., for her question:

What causes explosions?

Boom: A big bomb explodes in a city. Buildings are blown to bits.. Boom: A stick of explosives goes off in a quarry. Chunks of stone and rock go flying in all directions. Sometimes an explosion happens by accident. A boiler bursts. Cellar walls and doors are blown outwards. All these explosives have the same cause. Something needs more space ‑ needs it in a hurry and has the force to take it.

Steam needs more space than water. Watch a pot with a lid on as

it comes to a boil. The fire below is turning the liquid water into gassy steam. The steam needs more room than the water and spreads out to take it. It shoves to find a hole out of the pot, It finds a weak spot at the lid and pushed hard enough to lift it up. Bob, bob, bob goes the pot lid as the steam puffs out to get more room.

Gases, need more space than liquids and solids. The boiling water in the pot turns to gaseous steam slowly. The steam escapes a little at a time. But suppose the lid was sealed on tight and the steam could not escape. Bottled up, it would get strong and stronger. Soon it would be strong enough to burst the pot to bits. This is what happens when a boiler bursts.

Certain chemicals can turn to gas in a split second. One moment there is a solid stick of dynamite. Someone sets it off. The solid stick suddenly becomes masses of out rushing hot gases. The gas puffs out with mighty force and a thundering roar. It goes with such speed and force that it moves everything in its path. If the dynamite was buried in a solid rock, chunks of stone are tossed in all directions.

These explosive chemicals must be set off, or burned before they will do their work. Usually it takes but a spark to start them. Sometimes a severe jolt will do it. They must be carefully handled and set off by trained experts. Most of them do little damage if they explode on the ground in the wide open spaces. There the rushing gases can‑spread out without crashing into anything.

Many of these explosives are made from cotton waste. Gun cotton is cotton treated with chemicals, It burns freely If set on fire in an open space. But it explodes if it is set off inside a container. Smokeless powder is made of gun cotton treated with acetone and vaseline. This is the terrible explosive that fires most of the guns of war.

Glycerine is a by‑product of soap‑making.. It is also used to make explosives. With nitric acid it becomes nitroglycerine. This is a yellowish oil and very dangerous for even an expert to handle. When mixed with sawdust and. saltpeter it becomes dynamite,,. Dynamite is easier to handle, but even so, no one short of an expert should tamper with it