This document discusses how the human body develops from a single cell through cell reproduction and division. It begins with a single fertilized egg cell, or zygote, that undergoes repeated cell division to form a multicellular organism. As cells divide, they differentiate into various cell types but all originate from that original single cell. Throughout life, old and dead cells are constantly replaced as cells have a limited lifespan. The document asks and answers several questions about cell reproduction, types of cells, cell size, and cell number in the human body.