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	Building Blocks Tiny cells are known as the 'building blocks of life' because all living creatures are composed of them. It's staggering to think that the human body is made up of over 50 billion separate cells. Body cell   Previous: Back Book: Incredible But True Section: Incredible But True Chapter: Birds and Mammals