take two plastic bags/baloons/anything you can fill with air with your mouth with you to school
challenge the most athletic person in the room, and tell them that if they can blow it up faster than you then you will give them [thing they like] (chocolate, whatever)
watch as they struggle to inflate it with their mouths, and then fill it up in a few seconds with this simple trick:
hold the bag in front of your mouth, and blow into it from a distance, like in pic related. By doing this, you are using the air not to fill up the bag, but to create negative air pressure which causes air to flood into it, filling it up very fast.
practice this at home so you don't embarass yourself.
afterwards, you can do a presentation at whatever grade level you are in, i mean, appropriately complicated.
