Re: Why doesn't a balloon pop if you stick a pin in at the top?

This is a neat experiment that I used to do with my classes too. Try 
blowing the balloon up different amounts (or try filling it with different 
amounts of water, that's a bit safer and easier on the eyes and ears) and 
then poking it with a needle.  See if you can get it so that the balloon 
doesn't pop, but releases the pressure in a slow stream.

A balloon pops when you puncture it because the air you blow into it is 
pushing against the sides of the balloon.  If you blow enough air into it, 
it will push so hard that it stretches the sides of the balloon quite thin.  
This makes the balloon relatively "brittle" at the areas where there is the 
greatest internal pressure.  When you puncture it, the material can no 
longer take the stress and it ruptures (tears apart) and releases all of 
the air inside at once.  Unless you blow up the balloon almost to the point 
of popping, the very top and very bottom are areas where the balloon has 
not been stretched to it's limit.  If you look, you can usually see that 
the balloon is a darker colour there.  That's because the material there 
has not been stretched as much as at the sides.  When you puncture it 
there, because the material is more "relaxed," it just makes a small hole 
which lets the air flow out slowly.  We say that these parts have less 
surface tension than the sides of the balloon.  I hope this answers your 
question.