flow lift

Occasionally a situation produces an effect opposite to what you would expect. A description of such a situation follows.
 
Glue a thin piece of cardboard about 3 by 5 inches onto the end of an empty spool of thread. Use a pencil point to make a hole in the card where the hole is in the spool. Push a thumb tack through another 3 by 5 card at its center. A straight pin could also be used. Put the end of the spool with the card attached over the point of the thumb tack. Hold the other end of the spool to your mouth. Use one hand to hold the card with the thumb tack. Blow forcefully into the spool and leave go of the card as you do so. The card is not blown away. Instead it seems the harder you blow the better the card stays in place.
 
The usual way to explain this is to refer to Bernoulli's law which says that the faster a fluid moves across a surface, the less the fluid pushes on the surface. So the side of the card which has air moving across is pushed on less than the card's other side. Since the slow moving air on the other side is pushing the most, the card is held up. I do not know if there is a commonly accepted explanation for why fluids act this way.
 
Bernoulli's law is for a fluid moving along a surface. If the fluid's path is toward the surface then the forces are what you would expect: the greater the speed, the greater the push. In this demo air does move toward the bottom card, but only at its center. Most of the card has air moving along the surface.