«I hoped you would tell me.» The president rested on its single back leg, holding a stick thatg gave off green incense-smelling smoke.

«And so I will. Gravity.»

«Do not play with me, Beowulf Shaeffer. This matter is vital.»

«I'm not playing. Does your world have a moon?»

«That information is classified.» The puppeteers are cowards. Nobody knows where they come from, and nobody is likely to find out.

«Do you know what happens when a moon gets too close to its primary?»

«It falls apart.»

«Why?»

«I do not know.»

«Tides.»

«What is a tide?»

Oho, said I to myself, said I. «I'm going to try to tell you. The Earth's moon is almost two thousand miles in diameter and does not rotate with respect to Earth. I want you to pick two rocks on the moon, one at the point nearest the Earth, one at the point farthest away.»

«Very well.»

«Now, isn't it obvious that if those rocks were left to themselves, they'd fall away from each other? They're in two different orbits, mind you, concentric orbits, one almost two thousand miles outside the other. Yet those rocks are forced to move at the same orbital speed.»

«The one outside is moving faster.»

«Good point. So there is a force trying to pull the moon apart. Gravity holds it together. Bring the moon close enough to Earth, and those two rocks would simply float away.»

«I see. Then this 'tide' tried to pull your ship apart. It was powerful enough in the lifesystem of the Institute ship to pull the acceleration chairs out of their mounts.»

«And to crush a human being. Picture it. The ship's nose was just seven miles from the center of BVS-1. The tail was three hundred feet farther out. Left to themselves, they'd have gone in completely different orbits. My head and feet tried to do the same thing when I got close enough.»