Incredible But True Incredible But True Human World History Makers Cities Of The World Art And Music Sporting Headlines Plant Life Fish, Amphibians, Reptiles Birds and Mammals Creepy Crawlies Marine Life Restless Earth Earth's Secrets Changing Weather Seas And Oceans Comparisons Amazing Structures Machines And Man World Of Inventions Travelling Around Mysterious Universe	Girl Or Boy? Some oysters can change from male to female, or female to male, several times during their lives.   Previous: Back Book: Incredible But True Section: Incredible But True Chapter: Marine Life