One; after reflecting in the twilight on the merit of the previous bulb.
or
Just one, but he/she will spend hours checking for technical problems.
or
Humpty Trumpty wants Mexico to pay for it all.
or
One minds the train, one trains the mind.
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When it came time to pay, the skunk didn't have a scent, the deer didn't have a buck so they put the meal on the duck's bill.
or
It ran out of juice!
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Take regular water and just boil the hell out of it.
or
Two, one to call the electrician, and one to mix the drinks.
or