Bernoulli's law

 
Push a thumb tack through the center of a piece of 3 by 3 inch card. Cardboard from a cereal box can be used. Place the card on the end of a sewing spool so that the point end of the tack is in the center hole of the spool. That centers the card on the hole. Hold the card in place as you pull out the thumb tack. Push the pointed end of a pencil through the card at the tack hole. Glue the card to the spool. Push the thumb tack into the center of another card. Lay that card on a table with the tack point up. Put the spool over the tack so the two cards are together. The tack will keep the lower card from moving side to side. Blow forcefully through the spool. Lift the spool as you continue to blow. The bottom card is not pushed downward. The two cards remain together for the few seconds that you keep the air moving quickly.
 
Bernoulli's law states that for air moving along surfaces, faster moving air pushes less on a surface than does slower moving air. The air moving quickly over the bottom card actually pushes on the card less than the still air underneath that card. So the card is held up.
 
It seems amazing that air behaves this way. What could it be that causes this? I have not yet come across an explanation. If you are interested you can make a very similar demonstration that last about 10 seconds. The following 3 screens explain how. You will need a hair dryer, a couple of pie pans [the type made from aluminum foil], a couple of rubber bands, glue, tape, scissors and a few other common items.
 
Cut a full sheet of paper in half to get a piece about 8 1/2 by 5 1/2 inches. Fold up 1/2 inch along the bottom edge and cut slits to the fold line as shown.
 
Form the paper into a tube that fits on the exhaust end of the dryer. Attach the tube with a couple of rubber bands or tape. Cut a hole somewhat smaller than the bore of the tube in a stiff card. Center the hole with the tube's bore and glue the card to the paper tube. When dry trim the card at the ends of the tabs to give the tube a 1/2 inch flange. Use scissors to cut the sides off of one pie pan leaving a 6-7 inch flat disc. Cut a hole in the disc the same size as the hole in the card. Tape the flange of the tube to the disc with the hole.
 
Then cut the other pie pan in almost the same way as the first. But be sure to leave the four tabs as shown. These tabs are bent in such a way to keep the disc from sliding to the side. Do not cut a hole in this second disc. Put the discs together with the hair dryer aimed straight down. Turn it on high and let the air stream hold up the bottom disc.
 
Don't run the dryer this way for more than about 10 seconds at a time. Not as much air as normal flows through the dryer. So not as much heat is carried away. The dryer gets hotter than normal and can be damaged from getting too hot. If the dryer quits, let it set 10 minutes and then try it. A safety device in the dryer shuts it off when the temperature inside the dryer goes much too high. After things cool off the safety device resets. It may be asking too much from the dryer to do this more than a few times.