The world is full to bursting with living things, but what
makes them tick?
Life is what makes Earth special. No other planet yet discov-
ered has such an amazing diversity or sheer mass of life. No
matter where you go, from the coldest ice sheet to the driest
desert, you’ll ﬁnd life—though sometimes you’ll need to pack
What makes something alive? What’s the scientiﬁc deﬁni-
tion of life? We don’t even have a very good idea of where
to draw the line between living and nonliving, at least on
a microscopic scale. Not everything breathes oxygen, not
everything ages and dies, not everything reproduces in ways
we fully understand.
Even though much about life remains a mystery, our
understanding grows by the day. We’ve discovered such
amazing things as DNA, the mechanisms by which we age,
how plants are able to make food from sunlight, why some
animals are so large, and much more ….