Have you ever played with a puzzle?  If you have, then you know that all the individual puzzle pieces fit together to make one complete picture.  Living things, or organisms, work the same way.  Our bodies are made up of tiny pieces, and when the pieces are put together they make us!  We call these tiny pieces cells.


What type of puzzle do you think is made up of more pieces: a large puzzle or a small puzzle?  Large puzzles have more pieces.  What is made up of more cells: a large organism or a small organism?  If you said large, you are right!  Elephants are made up of many more cells than a very tiny mouse.  This makes sense, because elephants are much larger organisms than mice!


Bacteria are very, VERY tiny organisms.  Bacteria live everywhere!  They live in the dirt and on your skin.  They also live in your body and on your toys.  Bacteria only have one cell.  That is what makes them so tiny.  You must use a microscope to see bacteria.


Bacteria can be used to help make our food.  When they are added to milk, they can turn the milk into cheese.  Other bacteria help keep us healthy.  They live in our body and help break apart the food we eat into very small pieces.  The little pieces of food can then be used to make energy.  We use this energy to run, think, and move.

Just because some bacteria help us does not mean that we can forget about bad bacteria that could hurt us.  You should always wash your hands before you eat or touch food, especially after you use the bathroom.  This keeps bad bacteria from getting inside of our bodies and making us sick.


Dogs, people, and trees are animals and plants that are made up of many cells.  Organisms that are made up of many cells are called multicellular.  "Multi" means many, and "cellular" means made up of cells.  So when you put the two words together they mean "many cells."  Any living thing that is made of two or more cells is multicellular.


Some living things, like bacteria, only have one cell.  Any living thing that is made of one cell is called unicellular.  "Uni" means one, and as you just learned, "cellular" means made up of cells.  When you put those two words together, what do they mean?  If you said "one cell" you are correct!


A microscope is a tool we can use to see things that are really small.  When unicellular living things are very small and cannot be seen without using a microscope, we call them microscopic.  At dinner tonight you can say, "The microscopic bacteria in my tummy are breaking apart my food into tiny pieces."  You will sound so smart!  Your family might even want you to teach them what you learned today.

Now for a closer look

GcG, Public domain, via Wikimedia Commons

How do you make a peanut butter and jelly sandwich?  First you take a slice of bread, and then you spread peanut butter on one side of the bread.  Next, you take another slice of bread, and you spread jelly on one side of that piece of bread.  Finally, you take the two pieces of bread, and smash them together to make the sandwich.  Make sure the peanut butter and jelly are inside the sandwich!  Your sandwich contains two pieces of bread, peanut butter, and jelly.  When scientists want to say what something contains, or is made up of, they use the word composed.  Let's try it.  Your sandwich is composed of two pieces of bread, peanut butter, and jelly.

Yuuuuuuum PBJ

Renee Comet (photographer), Public domain, via Wikimedia Commons

Animals and plants are made up of many cells.  How can we change that sentence if we want to use our new word?  Animals and plants are composed of many cells.  How many cells are bacteria composed of?  That was a trick question!  The answer is one, because bacteria are unicellular.


What did we learn today?  We learned that living things are called organisms.  Living things that have two or more cells are called multicellular.  Living things that are made up of only one cell are called unicellular.  We can only see unicellular organisms by using a microscope.  One kind of unicellular organism is bacteria.  Finally, we learned that if you want to talk about what a living thing is made of, you could instead say a living thing is composed of cells.