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nytimes: the week in science, lead is a study that evolution may happen in much shorter time spans, given the right conditions.

The Human Body
Cells

The human body is made of cells. Cells are so small that you need a microscope to see them. Each cell takes in food and gives off wastes. There are about 100 trillion cells in the human body. Your body creates three billion new cells every minute to replace those that die.

Cells contain genes. Genes are units of heredity. Heredity refers to the traits we inherit from our parents. These traits include height, sex and hair colour. Genes are located on chromosomes inside the cells. Each body cell contains 23 pairs of chromosomes and about 100,000 genes. Genes are made of DNA. DNA is short for deoxyribonucleic acid.