Air pressure problem (gr.10)

hi!

i kinda grasp the concept of atmospheric pressure and how it affects everyday stuff... but i'm stuck with this one problem that i can't seem to understand.

The experiment involves filling a cup with water. Then, a cardboard piece is placed completely over the opening of the cup and the cup is inverted (with the cardboard held in place over the opening). As you may know, the cardboard becomes 'suctioned' to the inverted cup. I just don't understand WHY????

i know it has something to do with atmospheric pressure. My sketchy hypothesis is that pressure inside near the top of the inverted cup is lower than that of the pressure at the inside bottom of the cup. owing to the fact that air travels from an area of high pressure to an area of low pressure, i am *thinking* that the atmospheric pressure outside the cup is striving to get to that low pressure area inside the the cup thus, causing suction.

I would really, REALLY, REALLY appreiciate if someone could explain to me why this happens or to confirm whether or not my hypothesis is correct. Thanks a bunch :-)