As part of a science lesson for the day, Olivia blew up the balloon and then let the air go. The neck of the balloon wasn't able to vibrate because there was a tube in it.
The experiment shows the difference between being able to vibrate and make sound (like blowing up a balloon and then letting the air out which makes a sound) and not being able to vibrate and making no sound (which is what Olivia was doing).
This was a very hands-on way to learn about how the human body works. It's simple, but was very effective for the girls to understand something that could have been quite confusing if they just read about it.