Quoted from xsvtoys:Now here is a bigger question. WHY does water stay in the straw when you lower the straw into a glass of water, put your finger over the top, then lift the straw up? Very few get this one right. Hint: pretty much everything you google for the answer will not be right.
Simple: gravity pulls the water down out of the straw when the water level in the straw becomes higher than the water level in the glass. This creates a vacuum and sucks air in from the top of the straw to replace the water that was drained out the bottom. If you seal off the top with your finger the air can’t come in to replace the water and thus the vacuum keeps the water in the straw.