One more from Ratner’s Star (1976):
On the way home they sat in the next-to-last car. A tripping device failed to work and their train, braking late, ran into the rear of a stalled work train. Billy found himself on the floor of the car. Ahead was stunned metal, a buckled frame for bodies intersecting in thick smoke. Then there was a moment of superlunar calm. In this interval, just before he started crying, he realized there is at least one prime between a given number and its double. (p. 5)
What seven-year-old Billy has rediscovered is a variant of Bertrand’s Postulate.