3"x2" index card
See the Science Chapter for more instructions, hints, tips, and ideas.
This is a real fun demonstration because at first glance it doesn't look like it should work.
It demonstrates Bernoulli's Principle which states that as the speed of a moving fluid increases, the pressure within the fluid decreases.
To make the props for the demonstration, drill a 1/4" hole in the middle of the 2"x2" and sand the wood smooth. Push the straight pin through the center of the index card and tape it in place. Place the pin inside the hole and hold it in place. With the index card on the bottom, take a deep breath, and blow gently and steadily into the hole on top of the wood. Release the index card. The index card will float slightly below the 2"x2", but won't fall until you stop blowing.
The air that is flowing out the hole is deflected toward the edges of the index card. This air is moving faster than the air below the card, so by Bernoulli's Principle, there is less pressure above the index card. This lower pressure holds the index card to the wood.
To make a neckerchief slide, simply glue a PVC ring to one of the sides of the block of wood and place the needle in the top of the hole.