Question

how are you able to drink juice using a straw?

Answer 1

by sucking the drink into our mouth by straw

Answer 2

when you insert the head of the straw in the liquid, and when you suck, basically you are creating a pressure difference between the straw and liquid. Now, all liquids and gases have a tendency to equalize pressure in case of a difference. Hence the liquid, to equalize pressure in this case, goes into the straw and voila! you can drink using the straw.