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Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere on a surface. In this case, the surface is the water level inside the straw. When the straw is placed in the water without any obstruction at the top, the pressure inside and outside the straw is the same. Therefore, the water level inside and outside the straw is equal.

When you place your finger over the opening and lift the straw out of the water, you create a partially closed system. This closed system involves the air and water inside the straw and the atmospheric pressure outside. Since the air inside the straw cannot escape, its pressure remains constant as you lift the straw. Meanwhile, the atmospheric pressure outside the straw pushes the water level downwards.

When you hold your finger over the opening, you trap the air inside the straw, which maintains its pressure. As the atmospheric pressure outside the straw pushes down on the water surface, the trapped air inside the straw counteracts this force and prevents the water from flowing out. This is due to the equilibrium between the upward pressure exerted by the trapped air and the downward pressure exerted by the atmosphere on the water surface.

To further illustrate this concept, if you remove your finger from the opening, the atmospheric pressure would push the water out of the straw because there is no longer any upward pressure exerted by the trapped air to balance the downward pressure. Also, if you try this experiment at a higher altitude, where atmospheric pressure is lower, you will notice that less water stays in the straw, since the upward pressure exerted by the trapped air is greater than the downward pressure exerted by the atmosphere. In conclusion, the water stays inside the straw when you cover the opening and lift it out of the water due to the balance between the upward pressure exerted by the trapped air and the downward pressure exerted by the atmosphere on the water surface. This exercise helps demonstrate the impact of atmospheric pressure on a partially closed system.