⟩ As part of a school experiment girl was sent to the middle of a nearby city with instructions to collect a sample so that pollution levels could be measured. She was given a glass container with a removable but tight-fitting lid. Of course she noticed that the jar contained comparatively clean air from the school environment. How did she ensure that she excluded this air and retrieved an absolutely accurate sample of the city air?
The girl filled the jar with water at the school. When she reached the appropriate point at the city center she poured all the water out. What replaced it was a true sample of the surrounding air.
