When you first tried to ride a bicycle, you probably tipped over a lot.  Riding a bicycle can be hard because you have to fight to stay up right, without falling over to either side.  To help prepare for this, you may have started with a tricycle and then tried a bicycle with training wheels.

Once you learn to ride a bike, you never forget.

Bicycle riders need to control both their body and the bicycle.  If the bicycle tips over to the left or right, they will fall over.  It can be hard to keep the bicycle balanced.  To balance means to keep something where it needs to be, not moving too far in one way or another.  When you balance a bicycle, you need to keep it from falling too far to one side or the other.

Living things need balance too.  If it is a really cold day outside and you do not have a coat or sweater, how will your body stay warm?  As your body fights to stay warm, goosebumps might start to cover your arms and legs.  Those bumps make the little hairs on your skin stand up straight.  This helps to hold warm air close to your body in order to raise your body temperature.  You might also start to shiver.  When you are shivering, your muscles are squeezing over and over again, in order to try to make the inside of your body warmer.  If your body becomes too cold, your cells might stop working and some of them could even die!

If your body becomes too hot, how does it cool itself?  It might start by sweating.  Sweat comes out of your skin as water and uses the heat from your body to turn into a gas.  When the sweat evaporates from the surface of your skin it takes some of the heat with it.  Evaporation is when liquid water turns into gas or water vapor in the air.  You can check how this works by doing a simple experiment.  Wet the back of one hand with water and keep the other
hand dry.  Then take turns blowing air across the back of each hand.  The wet hand will feel colder because of the evaporation. 

Boy it's hot out here.

Since your body has different ways to both warm you up and cool you down, it can keep your body temperature in balance.  Homeostasis is when living things work to keep balance on the inside.  There are other ways that bodies do this. 
For example, when you are out of breath, you breathe faster and deeper.  When you are thirsty, your body tells you to drink water.

There are different organs in animals that help keep balance inside the body.  Plants also have different parts that help keep them in balance, too.  In people, the lungs will move air in and out, depending on how much air the person needs.  This helps to keep the amount of air in the body in balance.  When people either do not have enough water in their body or they have too much salt, a message will be sent from the brain that the person needs more water.  This will cause the person to feel thirsty, and it will also cause some of the organs to save water.  Most organs in living things have different jobs in order to keep homeostasis in the body.  A function is
a specific job that something has.  Many of your organs have one or more functions that help keep your body in balance.  The job of the leaves on plants is to soak in sunlight in order to make energy for the plant.

In a house, it makes sense for people to have different jobs.  It would take a very long time for each person to do every job that needs to get done.  Instead of everyone having to buy food, make food, clean the whole house, wash dishes and do laundry, different people might do one or more things.  When everyone does something different, every job gets done and people can do one job well instead of many jobs not very well.  One person might be better at cleaning, someone else at cooking, and so on.  Each person is specialized.  Specialized means being
very good at one or two things, instead of just okay at doing many different things.  Animals and plants work the same way.

Each different organ specializes in doing one job very well.  This makes sure that everything that needs to happen
to keep the animal or plant alive is always done.  When each part of the animal or plant has its own job to do, that part can be very good at its job.  The lungs specialize in bringing air into the body and plant leaves specialize in soaking up energy from the sun.  Our organs are able to be very good at their jobs because they specialize in one thing.  Each of these parts can work as hard as it needs to in order to keep different parts of our body in homeostasis.

Living things need to work hard to keep in balance, or in homeostasis.  If this does not happen, then an animal may breathe in more air than
it needs, or a plant might not soak up enough sunlight.  Your body is always working to keep balance, even when you are sleeping.  One of the reasons animals and plants can do this is because each organ or body part has a job to do.  These parts specialize in their own jobs, and that helps keep the organism working at its best.