When you drop a straw into a bottle of soda, that straw is filled with air. As you put the straw into your mouth and start sucking in, you begin by sucking the air out of the straw, creating a vacuum, or airless space, inside the straw.
Since air pressure is at work all around you, that air pressure pushes down on the soda in the bottle, forcing it up into the straw to fill that airless, or empty, space.
When the soda inside the straw reaches the same level as the soda outside the straw in the bottle, it waits for you to suck it up the rest of the way to your mouth.
