This nonfiction essay, first published in Vertex in April 1973, discusses the grandfather paradox.

Welcome to Ringworld, an intermediate step between Dyson Spheres and planets. The gravitational force created by a rotation on its axis of 770 miles per second means no need for a roof. Walls 1,000 miles high at each rim will let in the sun and prevent much air from escaping. Larry Niven's novel, Ringworld, is the winner of the 1970 Hugo Award for Best Novel, the 1970 Nebula Award for Best Novel, and the 1972 Ditmars, an Australian award for Best International Science Fiction.

This early Niven story was the first of many to delve deeply into the legal and ethical implications of organ-transplant technology. It introduces Known Space's organ banks and the breed of criminals known as "organleggers".

Nothing quite prepared telepath Larry Greenberg for mind-to-mind contact with an alien. In the interest of science, Larry tapped the mind of Kzanol - and that was his first mistake. Kzanol was a thrint from a distant galaxy. He had been trapped on Earth in a time-stasis field for two billion years. Now he was on the loose, and Larry knew everything he was thinking. Thrints lived to plunder and enslave lesser planets - and what the planet Kzanol had in mind was Earth!

This nonfiction essay, first published in Galaxy in March 1969, discusses the many uses, implications, and limits of teleportation, from personal transportation to interstellar travel.