'Frost and Fire,' and 'Uncle Einar' Summary

In 'Frost and Fire,' a boy called Sim is born in some caves. He watches his parents dying before his eyes, and is aware that he is growing unnaturally fast. He soon comes to learn that his people are the descendents of those who crash landed on this planet a long time ago. They were affected by the sun's radiation and now live for only eight days, growing up, maturing and growing old all within this short time. They have racial memory and a form of telepathy, so they are born with knowledge of their past. They cannot venture outside their caves except at dawn and sunset, as the sun's heat would kill them and the night-time cold would freeze them to death. Most of them spend their short lives...