Ballon experiment
This is a video which is the answer of the question on previous page.
As you can see, the answer is 2. Balloons pull each other.
Why does it happen?
![Picture](/uploads/2/5/0/7/25079019/1386495092.png)
Objects usually have pressure around it as shown in the left picture. However, when there is a movement of air, which is wind, near one side, the pressure decrease as shown in right picture. (Bernoulli's principle) However, the side that does not treat with wind has same amount of pressure as before. Therefore, the side with higher pressure moves the object toward the side with lower pressure. This is what the Bernoulli's principle is.
We can say the same thing for wings even though it's little bit more complicated. And what you learnt just now is answering the question, how do wings create a life force?