Frankie and his little sister Mollie went to a family birthday party recently, and everyone got a helium balloon as a favor. Mollie thought her big red balloon was just about the most wonderful thing she had ever seen. She played with it all the way home.

By the next afternoon, Mollie's balloon had gotten flabby and wrinkled. Instead of floating, it just dangled from the end of its string.

Mollie was very upset. "Fix it," she told Frankie. "Make it better," she begged her mom and dad. But they couldn't. The molecules of helium gas inside Mollie's balloon had sneaked out through its skin during the night.

"I'll blow up a new one for you," Frankie said. And he picked a bright blue balloon out of the family's party drawer.

But the balloon he blew up for his little sister didn't float. It looked great, and she had fun playing with it. She could make it bounce high into the air -- but it always floated back down to the floor again.

What's going on here? Why do some balloons stay airborne while others stay on the ground, even though they're light and bouncy?

The balloon Mollie got at the birthday party contained a gas called helium. Helium is much lighter than air, so a balloon filled with it floats. The helium inside the balloon rises, taking the sphere of rubber with it.

The balloon Mollie's brother blew up for her contained plain old air. It's still light, so you can bounce it around. But because the air inside the balloon weighs the same as the air outside the balloon, it will always fall back to earth again.

Someone used helium stored in a tank to blow up Mollie's birthday-party balloon. Frankie used air from inside his lungs to blow up the balloon at home. Both methods make the stretchy rubber that forms the balloon expand as air pressure inside it increases.

There is an easy way to find out how strong air pressure can be. You'll need a balloon that has been stretched a few times and a heavy book. First, lay the balloon on a table so that its neck is hanging over the edge. Second, place the book on top of the balloon. Take a deep breath, and blow air into the balloon. Can you blow long enough to make the book flip over?

You bet you can. When you blow into the balloon, the air pressure is enough to lift the book.

Have you ever seen a hot-air balloon floating through the sky? How, you may have wondered, does it stay up there?

Hot-air balloons float because of the way air behaves when it warms up. Air is made up of tiny particles called molecules. When the air is cold, the molecules crowd together. When it's hot, the molecules in air move farther apart. Because there's more space between the molecules, hot air is lighter than cold air. It rises.

Hot-air balloons ascend, or go up, because there's a propane gas burner in the basket, or gondola, attached to the bottom of the balloon. A blast from the gas heats up the air inside the nylon balloon, and it rises. As the air cools, the balloon gently drifts down again.

Balloons are a lot of fun, but they're useful, too. Some scientists have used hot-air balloons to get close enough to the tops of trees in the Amazon rain forest to study the communities of animals and plants that live in the upper branches, hundreds of feet above the ground.

Meteorologists -- scientists who study the weather -- send balloons high into the atmosphere to gather pieces of information up there. These big balloons are filled with a gas called hydrogen, and they tote along equipment that measures air pressure, temperature and humidity. There's a radio transmitter, too, so scientists on the ground can keep track of the balloon. Weather stations at different locations all over the world send up the balloons and share the information they record.

When a weather balloon floats higher into the atmosphere, the air pressure around it decreases. When this happens, the hydrogen in the balloon exerts more pressure on its walls. The balloon starts to expand, It expands and expands until -- POP! -- it bursts. The equipment it carries falls back to the ground. Luckily, it is attached to a parachute, so the scientific instruments land safely instead of crashing. Then scientists use the measurements the instruments gathered to learn more about predicting the weather.

Mollie isn't too interested in learning about the weather right now. She'd rather play with her balloon. "Catch," she says as she bounces the beautiful blue balloon to her brother.

Tips for Parents

Caution: Balloons are favorite toys, but they can cause choking if they are swallowed. Tell your children never to put a whole balloon or any pieces into their mouths. Dispose of burst balloons right away. Don't let children younger than 4 play with balloons without adult supervision.

But for lots of balloon fun, try "Balloon Science" by Etta Kaner (Addison-Wesley; $8.95). This hands-on science book for kids ages 7 to 12 contains 54 easy-to-do science experiments and comes complete with a packet of yellow, red, blue and pink balloons.

Catherine O'Neill is a children's writer.