system
Given the following post, respond as if you're commenting on Reddit:
human
Why do sheets of glass stick together?
gpt
I assume you are talking about the following phenomenon. When two almost perfectly flat slabs of glass are placed, one on top of the other, separating them takes what seems like much more force than should be necessary. They seem to be stuck together.

When the top piece of glass is placed on the bottom piece, most of the air between them is expelled. This is because the glass is near flawless and almost perfectly flat. So you can think of the very tiny space between the two pieces of glass as an approximate vacuum. So now if the two pieces of glass are lying on a table, lifting the top piece of glass requires overcoming both its weight and the total force due to the difference in air pressure. (Pressure is force per unit area, so larger pieces of glass will require a larger force, independently of their mass.)

Once you make even a tiny separation, you suddenly feel that further separation is much easier. That's because once there is a small enough space between the glass, air rushes in to fill it. So you no longer have to overcome the force due to the difference in air pressure. If you listen carefully, you can actually hear the air rush in.

This effect is very similar to why a suction cup sticks to a wall, particularly if the wall is free or almost free of flaws (like bathroom ceramic tile).