Jill Esbaum - National Geographic Little Kids First Big Book of Why 2

You sneeze, ah-CHOO! Your stomach growls, grumble-rumble.
Your heart races, lub-dub-lub-dub-lub-dub. Your body is
amazing, and theres a reason for everything it does.

Your one-of-a-kind voice is a remarkable instrument.
Your words help you communicate, of course,
but even the sound of your voice lets people
know if youre feeling happy or sad, excited
or angry, worried or afraid, and so much
more. Because no two peoples vocal cords
and mouths are exactly the same,
each has a voice unlike
anybody elses.

singing
works the same way
as speaking.
Each of us has a
singing voice unlike
anybody elses.
la-la-la!

The
faster a
persons Vocal
cords vibrate,
the higher their
voice will
sound.

Your voice starts with a huff of air coming up from your lungs.
The air moves across your voice box, called the larynx (say
LARE-ingks). In your larynx are folds of thin skin called
vocal cords. When air passes across the cords, it causes them
to move back and forth, or vibrate. This makes sound, which
goes into your mouth.

Your mouths size and shape, as well as your tongue, your lips
and cheeks, and even your teeth, work together to create a
voice that your family and friends instantly recognize as YOU.

A dogs bones and
muscles are made to
move more quickly than
ours. Scientists say our
bodies might be able
to move at speeds of
up to 40 miles an hour
(64 km/h)but nobody
has come close to that yet.

If you want to win a foot
race against an animal,
challenge a chicken.
Its top speed is only
about nine miles an hour
(14 km/h).

Have you ever
run in a race?

The
worlds
fastest dog
is the greyhound.
It can run 43
miles an hour
(69 km/h).

The
worlds
fastest human is
currently usain bolt.
He ran more than 27
miles an hour
(43 km/h) for a short
distance.

Like other
muscles,
hearts get
stronger
with regular
exercise.

Your heart is a muscle that pumps blood
through your body. That blood has
important jobs to do. It carries oxygen to
every part of you, along with everything
else your body needs to stay healthy. It also
carries away waste.

When you exercise, your body works harder. It needs more
oxygenfast! Your heart speeds up, pumping harder to keep
blood flowing to where its needed.

Facts

Your heart is the size
of your fist.

Your rib cage pro
tects your heart.

Of all the muscles in
your body, your heart
works the hardest.

Your heart beats
about 100,000 times
a day.

Youll need

a stopwatch
or a watch
with a second
hand

paper and
a pen or
pencil

Sit down,
then push
your fingertips
against the front
of your neck, just
to the side of
your windpipe.

Count how
many beats
you feel in one
minute. Write
down that number.

Do 20
jumping
jacks, then count
the beats again for
one minute. Write
down that number.

Did your
heart beat
faster or slower
after exercising?
Rest one minute,
then count again.
Has it slowed down?

Nobody knows why blushing
happens. But embarrassing
moments happen to all humans.
That everybodys looking at
me feeling can make your
cheeks feel hot. You know
theyre getting red, red, RED.

What kinds of things make
you feel embarrassed?

That redness is because of tiny
veins just under your skin.
For some reason, when you
feel embarrassed, your brain
tells those veins to open wider.
Blood floods into them, and
your skin looks red.

blood
vessels are
tubes that carry blood
throughout your body.
Some are as big around
as an adults thumb.
Others are smaller
than one of your
hairs.

scientists
say that telling
somebody theyre
blushing even
when they are not
can make them
blush.

Imagine you bump your kneeow, that hurt!
Theres no cut, but small blood vessels under your
skin were damaged. A little blood leaked out of
them. That shows through your skin as a red area.

But dont worry. Your body is already working to
heal itself, to break up and reabsorb that blood.
Soon that red bruise turns bluish-purple. Over the
next few days, it turns greenish-yellow, then light
brown. In a week or two, youll check the bruise
andhey, whered it go?