Cut a hole in the center of the index card just large enough to stick the straw through.
Put the index card flat on the table. Put the straw into the hole.
Holding onto the straw, tape it to the index card so that it won't move.
Bend the paper clip so that one end is pointing up. Tape the paper clip onto the center of another index card with the end of the paper clip pointing up.
Put the straw and index card over the point of the paper clip, turn the whole thing upside down, so the index cards are on top of the straw, and blow through the straw. The card doesn't blow off the straw.
Turn the straw and cards over so that they're on the table and you're blowing down. Pick up the straw and top index card. If you keep blowing, both of the cards come with you.
Here's why this phenom works: First you have to remember that there's air all around us and it pushes on everything in all directions. Air is pushing up and down on all sides of these cards.
Then you have to think about what happens when you blow through the straw. When you blow through the straw you're making air move quickly toward the bottom index card.
But before the air reaches the bottom card it moves really fast and is able to slip through the little space BETWEEN the two cards. Because the air slips in between the two cards, it doesn't have enough force to push down on the bottom index card.
Now remember the other air that's pushing on everything in the room, well that air is still pushing along the bottom of the card. And because there's not much air pushing BETWEEN the two cards, the air along the bottom is able to push up and hold the two cards together.