Need help on a short quick meteorology lab project?

It is essential that you have no air in a gap between the rigid plastic lid and the liquid; the container must be completely filled with water. While holding the lid in place with your finger, turn the vessel upside down. Then remove your finger. The reason that the lid remains in place after you turn the vessel upside down is that the pressure of the atmosphere (at about 14.7 lbs per square inch at sea level) is equal at every point on the vessel.