Question

How the straw works in sucking liquid

Answer 1

There is no such definite force.
When you pull air out of the straw into your mouth, you lessen the atmospheric pressure inside the straw and create a void in it.
This void cannot remain like that... it must be filled.
​Since, the outer atmospheric pressure is more now than the atmospheric pressure inside the straw... the outer pressure forces the liquid into the straw to fill that void . That is how the liquid goes into the straw.