Science can push and pull, but it will never suck anything. The key is the concept of pressure, pressures like to always be equal and will try just about anything that there is to get that way. This desire can produce those pushes and pulls, but once again, it will never suck. Read the following for more...
they simply are a tool used to equalize pressure. When you drink through a straw you are simply creating an area of low pressure in your mouth. You do that by sealing off your throat with your tongue and then moving your tongue backwards. This creates greater a volume in your mouth with the sam amount of gas (air), causing a low pressure. The atmospheric pressure outside becomes greater than the pressure in your mouth and the fluid will be PUSHED up the straw into your mouth until the pressures are equal or you open your mouth to the atmosphere again.
The pressure on the inside of the cup is reduced almost to zero when the cup is pushed against a smooth surface. The atmospheric pressure is much greater than the pressure in the cup so it PUSHES in vain to try to equalize, it can not so it provides a very tight seal between the wall and the suction cup.