The Characteristics of Life: Six Signs of a Living Thing

0:00Which of these is alive: a dog, a tree, a candle flame or a growing crystal?

0:08Biologists check six signs of life. First, living things respire: they release energy from food.

0:16Second, they grow: they get permanently bigger.

0:21Third, they take in nutrients, the substances they need to live and grow.

0:27Fourth, they produce waste, like carbon dioxide, and get rid of it.

0:32Fifth, they respond to stimuli, which are changes around them, like light or sound.

0:39Sixth, they reproduce, making more living things of their own kind.

0:44And all living things are made of cells, the tiny building blocks of life.

0:50The dog respires, grows, eats, makes waste, comes when called and has puppies. It is made of cells, so it is alive.

1:01The tree respires, grows, takes in minerals, gives off oxygen as waste, grows toward light and makes seeds. It is made of cells, so it is alive.

1:13A candle flame gets bigger and uses oxygen, so it seems to grow and respire.

1:19But it is not made of cells. It is just burning, a chemical reaction, so it is not alive.

1:27A salt crystal seems to grow, as more salt builds up on its outside.

1:32But it does nothing else on the list, and it is not made of cells, so it is not alive.

1:39One or two signs are not enough. Living things are made of cells and show all six signs.