# How does water fill an upside down cup?

On Friday before leaving work, I left one of those smooth carton cups with a lid filled $$1/2$$ with water on my desk. The desk is made of wood without any special coating. Beside the carton cup, I also had a small plastic water cup upside down.

When I returned on Monday, only about $$1/4$$ of the water in the carton cup remained, while the desk below had absorbed quite a lot and was still soaked. This all makes sense as the water was probably slowly leaking out of the carton cup.

But what interests me is how the small upside down plastic cup beside it was also filled with about an inch of water. How did the water enter and fill one inch of the plastic cup?

Edit: More specifications, the temperature in the office is always at 72 degrees Fahrenheit with about .3 margin of error below. There is also no sign of water spreading around the cup except for the darker wood color that is still wet. The plastic upside down cup has been there for a week or more unmoved, therefore I am unsure if the temperature inside the cup differed at all from the room.

I also had a napkin beside the cups, but I didn't mention it as I did not think it had much to do with this phenomenon. As you would expect, it is also damp.