Materials required: - a glass tube or an ink filler, or a straw. Take one of the above tube. Dip it in water and suck water carefully from the top end. Let not water rush into your mouth. Close the top end with your forefinger immediately. Lift the tube above the water. Does the water in the tube fall down? Gently release your forefinger.
How does water flow down? Lift your finger as you please and let the water flow down. This kind of tube is called the pipette used in laboratories and pharmacies. You can measure various quantities of liquid using the pipettes. The pipettes are available in different sizes.
Why does the water flow down the tube, when you remove the finger? When you lift the tube, air at the bottom of the water column lifts it up. But when you remove your finger and allow the air to enter from the top, air presses the water column from the top. Air pressure at the top and at the bottom is the same. But due to the weight of the column of water it flows down.