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1. In the beginning there was a great Mathematician who invented the sphere - a massless figment of his imagination.

2. Along came a Physicist who had been thinking heavily about mass. He filled the sphere with a perfect fluid, but being an experimental fellow he decided to rotate it at a uniform speed. The points at which the axis of rotation crossed the surface he named the N pole and the S poles. The line on the surface which was equidistant from the poles he called the equator. He observed that the sphere became flatter at the N & S pole. The new shape he called an ellipsoid.

3. The physicist took an apple and weighed it on a spring balance. He discovered that the apple weighed more at the poles than at the equator.

4. Knowing that the the equator was further from the centre of object than the N-pole, he decided to see how much energy was required to roll the apple on the surface from the N-pole 'up' to the equator. This proved to be very easy because he did NO WORK at all. In other words he needed no energy, not even a bite from the apple!

5. No work had been done to the apple and yet its weight had changed! The physicist explained this very curious result by saying that the centrifugal force created by the ellipsoid's rotation had helped to move the apple further from the center of mass and against the force of gravity. He needed a new word to describe this surface which possessed different gravities but which had no 'uphill'. He called this surface equipotential.

6. A Geologist arrived on the scene, but thinking that all new words should begin with "geo", he coined the word geoid for the equipotential surface.

7. The geologist amused himself by chucking some big lumps of rock into the rotating body and so created local anomalies. The geoid was no longer an elegant mathematical shape, but it had bumps in the surface. The physicist could still roll his apple over all the bumps without consuming or gaining any energy. However the equipotential surface was no longer identical to the ellipsoid.

8. Next came the Chemist who thought that the body would look much nicer covered with a solution of sodium chloride, but because he had not stirred the solution very well, the liquid was not of uniform density. This meant that the liquid surface did not fit the geologist's geoid exactly. The new surface became know as the Chemist's Level (or C-level for short).

9. It was now the Meteorologist's turn. He added winds, temperature differences, and lots of other nasty things. This changed the shape of C-Level to the Meteorologist's Special Level (or MSL for short).

10. Not to be outdone, the Oceanographer, who had been circulating, chipped in with the observation that the sodium chloride solution was in constant motion. He called this movement 'Ocean currents' and found that this changed the MSL in an even more complicated way.

11. And the last to appear was the Environmentalist with his dire warnings of global warming. He talked of melting ice and great changes to the MSL. He said that in 100 years time everyone else would be wrong anyway!

12. The geologist, having become bored with the conversation, went outside to inspect his plates. When he returned a few millennia later he proudly announced that whilst he had gone up in the world, all the others had gone down.

That explains why the seven level headed scientists are not the same height.

And if you want to know your height, it all depends in whose shoes you stand and when you stand in them.

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