Suction cups are strange when you think about it. It's just a curved piece if plastic but it can stick to walls. So what makes it so different that it doesnt just fall from the wall? Normally if you try to stick an object to a wall, it just gets pushed away by a normal force. However in the case of a suction cup, when you press it against a surface, the air underneath it is pushed out and creates a vacuum, creating a difference in air pressure. This causes air to apply a force that keeps it against the surface, and friction prevents it from sliding down.