If you have ever played with balloons, you know how easily they pop. Any sharp object that touches them could produce a loud bang, and lots of pieces of shredded balloon. In this activity, we are going to stick a sharp stick completely through the balloon without popping it.
You will need:
- some balloons - 9 inch or larger will work better
- one of the bamboo skewers that grocery stores sell for grilling
- cooking oil
- a piece of string about a foot long
Blow up a balloon and tie it off. Dip the bamboo skewer into the cooking oil, and let the excess drip off. Now, you are going to stick the skewer into the balloon. You may be expecting a bang, and if you just randomly stab it, that is probably what will happen. Instead, look at the top of the balloon (the end away from the opening that you blew into.) You will see a thick spot in the rubber of the balloon. This is where you should stick the skewer into the balloon. Place the point of the skewer in the center of the thick spot and push firmly. It will puncture the balloon, without popping it.
You can stick the skewer all the way through the balloon by moving the point to the thick part of the balloon where you tied the knot. Push the skewer firmly and it will puncture the rubber again. Now, the skewer is stuck all the way through the balloon. You can even pull out the skewer, and the balloon will not pop, although it will slowly deflate as the air goes out through the holes.
Why did the balloon not pop? Rubber stretches because it is made up of long, twisted molecules. Get a piece of string about a foot long. Tie one end to something fairly heavy. Then start twisting the other end. As you twist the string more and more, it will start to bunch up, with the string winding around itself. When you have several bunched up places on the string, pull on the string. You will find that it stretches longer as the twists unwind. When you stop pulling and give it some slack, the string will bunch up again, getting shorter.
Now, imagine that your balloon was made up many, many twisted strings. That would explain why the rubber stretches. If all the strings were bunched up, then a sharp object would tend to slip between the strings, with them stretching a bit to let the point through.
On the other hand, if the strings were stretched tight, then the sharp point would tend to break them, causing them to snap back and unravel. When the rubber of the balloon is stretched tightly, that is exactly what happens, causing the balloon to rip.
You can explore this by using an uninflated balloon, or part of a popped balloon. If you stretch the rubber tightly, and then pierce it with the skewer, it will tear, but if the rubber is not stretched, it just makes a hole.