A push in two directions

Blowing up a balloon takes a lot of huffing and puffing. But if you let
the balloon go, the air rushes out—and the balloon takes off like a
rocket! What makes it fly?

When you blow up a balloon, you push air into it. The force, or push, of
the air inside the balloon makes the balloon bigger.

But anytime something makes a push, the thing it pushes against pushes
back. The two pushes are exactly the same size, but they go in opposite
directions.

The air inside is pushing on the balloon, but the balloon is pushing on
the air, too. When you let go, the balloon pushes the air out of the
opening. But the air pushes back at the balloon—and the push makes the
balloon fly.

A rocket balloon

This project proves that when there is a push in one direction, there is
always an equal push in the opposite direction.

Materials

balloon (long)
crayons
drinking straw (long)
paper bag (small)
string (long)
tape

Flatten the bag. Place the straw on the middle of one side, along the
length of the bag. Tape the straw to the bag. Decorate the bag to look
like a rocket, using the side with the straw as the top.

Thread the string through the straw. Then stretch the string between two
chairs. Tie the ends so that the string is tight. Slide the straw to one
end of the string.

Slip the balloon into the bag. Then blow up the balloon. Now your rocket
is ready to fly.

Let go of the balloon. Your rocket will take off and zip forward along
the string, pushed by the air rushing out of the balloon.