system
Given the following post, respond as if you're commenting on Reddit:
human
If I put oil between two flat plates, why is it harder to pull them apart? What is this pressure called?
gpt
If the flat plates are very flat and aligned, and it is hard for air to get in between the plates because of the oil, then the pressure holding together the plates is air pressure and is called "suction" or simply "air pressure". Gravity pulls down on Earth's atmosphere, causing the air to push strongly against all objects it can reach. Usually, air pushes on one side of an object equally as strong as on the other side, so that the force from the air cancels out and you don't notice it. But if you get rid of most of the air on one side of an object and block the air from getting back to that side, then there is nothing canceling the air pressure on the other side of the object. As a result, the object feels a net force from the air pressure. 

This is operating principle behind suction cups and vacuum cleaners: remove the air on one side, and air on other side pushes the objects. But for this effect to work, you have to keep air from leaking to the one side and balancing out the air on the other side, i.e. you have to maintain suction. For instance, slide a pin between a suction cup and the wall, and you provide a way for air to leak behind the suction cup, and it will fall off. The key is no leaks, which requires an air-tight seal. The oil between the two plates pushes the air away and then provides a seal for air to not leak back in. In trying to pry apart the plates, you are therefore fighting against about 100 km of air sitting on top of the plate. To best way to get the plates apart is to break the suction by wiggling a knife between the plates at one location to help the air leak in.

There are also molecular forces at play pulling the oil and plates together, but these are very weak compared to air pressure.