In this experiment, when we place a balloon in the bottle, we try to inflate it at the beginning. The balloon will not swell because the area covered by the air does not allow the balloon to swell. Then you will be able to see the balloon swollen when you drill a small hole on the bottom of the bottle. After the balloon swollen, close the hole immediately. Thanks to the hole we opened, we could inflate the balloon because the air inside the bottle could go out. Do a small show before you show your little secret. Finally, don't forget to make a scientific explanation in accordance with their age.
