What to look for
A common toilet plunger will be used to stick to a clear piece of plexiglass. The clear plexiglass shows that there is nothing in the plunger, but it is "stuck" to the plexiglass.
How it happens
The reason this happens is because the air is forced out from between the plunger and plexiglass and when the plunger tries to straighten up again, there is a low-pressure area created under the plunger, and the higher atmospheric pressure pushes the plunger onto the plexiglass.
Thus, the plexiglass and plunger are squeezed together by atmospheric pressure. The usual term used is that the plunger is held to the plexiglass with "suction". There is no such thing as suction, only a lowering of the pressure in one place or another.
When your vacuum cleaner cleans up dust or dirt, it does not "suck up" anything. It creates a lower pressure (vacuum) in the hose, and the higher atmospheric pressure pushes the dust or dirt into the hose and on into the container in your cleaner that accepts the dirt and dust.
atmospheric pressure: This is also the pressure caused by air, but usually thought of as the normal value or standard value of 14.7 pounds per square inch. The atmospheric pressure at the place where we are on the face of the earth is usually less than this because we are above sea level, and hence have less "weight" pushing down on us. Again, used with the same activities as air pressure.
low pressure: A space where the pressure is fewer pounds per square inch than a neighboring space. We will use this term along with the term high pressure in order to describe the resulting force that will tend to push something one way or the other.