$10-$d$1$ Adrinking straw works because, when you suck air out of the straw, there is decreased pressure. The equilibrium water level inside the straw is such that the pressure at the water level outside the straw $($atmospheric pressure$)$ is equal to the pressure at the same height inside the straw. Inside the straw, there are two contributions: the air pressure inside the straw plus the column of water above this height.