Question

how is it possible to suck up lemon soda through a straw

Answer 1

When you suck air from your mouth you are actually reducing the air pressure at your mouth and the mouth is connected to the lemon soda through straw. Now lemon soda is exposed to the atmospheric pressure and hence has higher pressure than the mouth. So the force acting on the lemon soda du to atmosphere is more compare to the force from the mouth. Hence the lemon soda moves to your mouth.

Answer 2

When you suck a straw you sallow some of the air from the straw so, the outside air pushes the lemon soda into the straw.