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Possible interpretation: If you want to heat water until it boils, and you watch it while you wait, then it seems to take a very long time. In the same way, anything that we wait for with eager attention seems to take a very long time: like waiting for someone to arrive, waiting for the phone to ring, waiting for a letter to come.

Note: watch (verb) = look at; observe with attention | watched = looked at; attentively observed | pot (noun) = a container for cooking or storage | boil (verb) = reach a very high temperature (at which water turns to vapour) | Alternative version: "A watched kettle never boils."