That's a great question :)
Before we answer this question, lets look at an
example of a cloud. What does a cloud do
besides moving in the sky? It rains! How
does cloud make rain and make rain fall down?
When water becomes really warm, it evaporates up
into the air to the clouds. As we get more and
more water evaporating up into the air, the
temperature of the water becomes colder and
colder, until water "condenses", or fall
down into the land as rain. This is because
cold air sinks while hot air like water that
evaporated rises up into the air. This process
is similar to your question. The air in the cup
is colder than the air surrounding the cup.
The surrounding air gets colder and colder till it
cannot hold any moisture like it used to, forming,
drops of water on the outside of the cup :)
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