Like everything else, the balloons are surrounded by air pressure. When I blew between them with the straw, I changed the pressure. Either the air between them stopped pushing as hard, or the air on the outer sides began pushing harder. Which do you think happened? As air squeezed between the balloons, it sped up, and lost pressure, and stopped pushing as hard. So the higher pressure of the air on the outer sides of the balloons pushed them together.
What would happen if you blew along the outer side of one of the balloon? When I blew on the outside of the balloon that created low pressure on the outside of the balloon and the balloon push to the outside from the high pressure in the middle.
So why did the paper lift up when I blew over it? How did that change the air pressure? Air never pulls or sucks; it only pushes. It pushes on you every second, from every direction. This constant push is called air pressure.
Did I change the push of air on the top or bottom of the paper? When I blew over the top, the moving air had to squeeze between the paper and the air above it. As the air squeezed through, it sped up, lost pressure, and stopped pushing as hard. The still air below the paper had greater pressure and pushed the paper up.
Just like on the highway/freeway and a truck passes you. Your car gets pulled into the middle to the passing truck. It is because the truck created low pressure between your car and the truck and the high pressure on the outside pushed your car to the truck.