Do this activities before you going to known the reason.

Take a straw. Drink something from it to be sure that it works. Now make a small hole in it (make sure the hole is above the level of the liquid in the glass). Does the straw still work?    

Working principle of strew.

When you sit down to a glass of iced tea with a straw in it, you might say that you are sucking the tea up through the straw. In fact, a more accurate description would be to say that the atmospheric pressure is pushing the liquid up the straw.

Decreasing air pressure:

With the straw just sitting in the glass, the pressure on the surface of the tea is the same all over, including on the little bit of surface inside the straw.

When you suck the air out of the straw, you decrease the pressure inside the straw, allowing the higher pressure on the rest of the surface to push the tea up the straw and into your mouth.

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