There is this farmer who is having problems with his chickens. All of the sudden, they are all getting very sick and he doesn't know what is wrong with them. After trying all conventional means, he calls a biologist, a chemist, and a physicist to see if they can figure out what is wrong. So the biologist looks at the chickens, examines them a bit, and says he has no clue what could be wrong with them. Then the chemist takes some tests and makes some measurements, but he can't come to any conclusions either. So the physicist tries. He stands there and looks at the chickens for a long time without touching them or anything. Then all of the sudden he starts scribbling away in a notebook. Finally, after several gruesome calculations, he exclaims, 'I've got it! But it only works for spherical chickens in a vacuum.'
My physics class the other day reminded me of this joke.
We were talking about Feynman, because at one point he said that if two humans each contained one percent more electrons than protons, when standing at arm's lenght their force of repulsion would be enough to lift the earth.
Well of course we had to calculate this to see if it was right.
To make a long story short, we figured out a good approximation.. if all humans were 100kg spheres of H20.