When you immerse a straw in water, the water flows into the straw and the level will be the same as the level of water outside the straw. If you block the hole of the straw, you can pull out the straw and the water will be 'stuck' in the straw as long as one end of the straw is blocked. When you release it, water will soon fall down. I can guess that this has to do with difference in pressure, but not sure how it actually comes about. Can someone enlighten me?