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Air pressure or Atmospheric pressure is the pressure exerted by the atmosphere or the air column above us. Air pressure affects us in so many ways. For example: A decrease in air pressure makes it more difficult to breathe and also to cook food. This is because Atmospheric pressure decreases with altitude. That is why it is more difficult to cook food and to breathe on the highest mountain (mount Everest).
If a small hole is made on the top of a tin of condensed milk, air inside the tin pushes the milk from inside, while the air outside the tin pushes the milk from the outside. The milk will come out fast enough initially and block air from entering inside. Air pressure inside the tin starts to get smaller. The pressure from the air outside will be relatively greater to hold the milk in against gravity.
This will cause the milk to drip very slowly or not at all.
If a second hole is made, then air will be able to get in the hole and keep the pressure balanced. Then gravity will pull the milk out, causing the milk to flow smoothly.
■ Make a small hole at one edge of the tin using a knife.
■ Tilt it toward a cup and observe what happens. The milk drips slowly and stop as explained above. Be careful when using a sharp knife. Adult supervision is recommended.
■ Make a second hole at the edge opposite to the first using a knife.
■ Tilt it again toward the cup and observe what happens. The milk flows smoothly as explained above.