The primary difference between living things (plants, animals, and bacteria) and non-living things (rocks, wood, and water) is that living things contain Instruction Books that tell them how to respond to changes in the outside world. Go outside at night and shine a flashlight on the sidewalk. The sidewalk doesn't do anything. Shine the light on a moth, and it will move towards it. Shine the light on a raccoon and it will flee. Many plants will turn towards the light, if you hold it on them long enough. The main difference between a living, growing tree and a wooden chair is that the chair no longer has a working Instruction Book.

Living things' Instruction Books change from generation to generation, due to mutation and selection. Try to swat mosquitoes and you select for faster and more agile ones. Watch cats chase mice -- they're selecting for each other, producing both better cats and better mice. With both the mosquitos and the mice, you're watching natural selection as it takes place. Grow some flowers and plant seeds only from the pretty ones. Go to the grocer and look at all the varieties of apples available. Both of these are examples of artificial selection.

Over time, mutation and selection produce variety in living things. Look at people passing on the street: the differences in skin color, eye color, height and other traits are due to their different Instruction Books. Look at flowers growing in a garden -- the differences between their colors and sizes are also due to differences in their Instruction Books. Even the differences between people and grass are caused by differences in Instruction Books. Variety -- diversity -- is good, as it reduces the chance that a single outside event (disease, drought, or famine) will kill off an entire species. Variety gives a species several many Instructions for responding to the event.

The next time you see a living thing think of its Instruction Book and how it makes the traits you see. Think as well about your own Instruction Book, which lets you see and think about Instruction Books.