The Last Drop

When Charlie retired from the waterworks, they gave him a gold-plated watch and a cardboard box to put his stuff in.
He took everything home but a large half-empty bottle of poison, which he left in the middle of his desk.
Charlie had started every day with a fresh cup of coffee, walking to the Filtration Pump Room, and putting a drop of poison in the city’s water supply.
He figured it would toughen people up a bit in these difficult times.
Charlie also dumped his coffee into the city’s water supply, but that’s because the coffee was so bad.