Years later, in a high-school biology class, Samuel heard a story about a certain kind of African turtle that swam across the ocean to lay its eggs in South America. Scientists could find no reason for the enormous trip. Why did the turtles do it? The leading theory was that they began doing eons ago, when South America and Africa were still locked together. Back then, only a river might have separated the continents, and the turtles laid their eggs on the river's far bank. But then the continents began drifting apart, and the river widened by about an inch per year, which would have been invisible to the turtles. So they kept going to the same spot, the far bank of the river, each generation swimming a tiny bit farther than the last one, and after a hundred million years of this, the river had become an ocean, and yet the turtles never noticed.This, Samuel decided, was the manner of his mother's departure. This was how she moved away - imperceptibly, slowly, bit by bit.