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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?

I remember reading something about this, but I have no idea what to Google or what question to look for here that answers this.

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.

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Did the temperature in the room change? Was it slightly colder that Monday morning, compared to the Friday afternoon? Or was there something hot inside the upside-down cup, which now has cooled down?

If the water spread out over the table and covers the rim of the upside-down plastic cup, then no air can enter. As the room temperature decreases, the air temperature inside the cup decreases since the cup is not thermally insulating. Colder air compresses and causes a vacuum effect with the pressure dropping inside the cup, according to the ideal-gas law. This might have "sucked up" some water.

If this hypothesis is correct, you should be able to see different water heights inside the cup at different times during the day.

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  • $\begingroup$ The temperature remained constant, as far as I know as it is controlled within the whole building to be optimal for working. There was no water visible around the cups, only the darker wood of the desk was eye catching as it was still wet. Also the water height seems to also remain constant. $\endgroup$
    – Manu
    Jul 16, 2019 at 18:04

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