Imagine that you are a scientist in the very early days of civilization.
You observe all kinds of objects around you, and you begin to wonder
about their similarities and differences. You wonder about the differ-
ence between yourself and these objects. You know that you are alive.
What does this mean? How many of the objects pictured here are alive,
and how do you know?
Movement is one of the signs that something is alive. Is it always a
sign of life, though? A rock rolling down a hill moves, and it is certain-
ly not alive. Perhaps you said that growth is a sign of life