After the cells have burned the food scraps, they turn the "ashes" and
"smoke" back into the blood-stream that is always flowing past them.
If the cells did not do this, they would soon smother to death, just
as you could not possibly live in a house without chimneys to carry
off the smoke. And, of course, the blood wants to get rid of this
waste just as quickly as possible.

Part of the waste in the body is liquid, like water, and can flow away
through the blood pipes without needing to be burned. Some of this
watery waste comes out through the skin and stands in beads or drops
upon it. That is the part we call perspiration, or sweat. The rest of
it goes in the blood to another strainer called the _kidneys_, passes
through this as _urine_, and is carried away from the body as the
waste water from the bathtub and the sink is carried away from a
house.

For the "smoke" Mother Nature has still another beautiful plan. She
sends the blood-stream flowing through the _lungs_, where it can send
off its "smoke" and then get fresh air to carry to the cells in the
muscles. When you breathe out, you are sending out the "smoke"; and
when you breathe in, you are taking in fresh air.

Our body "smoke" is not brown or blue, like the smoke from a fire; it
is a clear, odorless gas, called _carbon dioxid_. This is the same gas
that makes the choke-damp of coal mines, which suffocates the miners
if the mine is not well ventilated; and the same gas that sometimes
gathers at the bottom of a well, making it dangerous for anyone to go
down into the well to clean it. And this gas is poisonous in our
bodies just as it is in the mine or the well.