Farmer smith was not satisfied with the yield of his milk cows, so he decided to called in an animal psychologist, an engineer and a physicist to try and improve matters. All three inspected the farm and the cows and made there recommendations.

The animal psychologist went first, If you paint the milking shed green the cows will be happier and happy cows will give more milk.

Then came the turn of the engineer. If you narrow the milking stalls by 10 centimetres you will be able to add an extra stall and thus be able to milk an extra cow in the same time.

Farmer Smith was very happy so far, now it came to the turn of the physicist. He got out a black board and started drawing an elaberate diagram. Then he started to talk:

First, consider the Cow as a sphere, radius r.

You probably have to have studied physics to get the joke!