How would you explain cellular respiration to an eight year old?

use pictures and illustrations and use examples of something they do know so they can imagine what happens..that age range is pretty smart as well...could see what they already teach kids at that age in sciences so that way you know where to start when you have to use certain terminology

It isn't impossible. Obviously it is possible if your teacher gave you the project.
You people are just so lazy. I'm sure that you just procrastinated until today - which happens to be Sunday, and it's due tomorrow right?

Try making err, pictures that are large and colorful. Try to bypass the large words and just explain as simple as you can "e.g. Your cells need food as well, and this is how they can get them!". Good luck. I woulden't even go near that subject if it was me. lol

Humans and other animals can't make their own food, so they have to get energy from plants. Cellular respiration changes the food that plants make so we can use it for our bodies.

ouch, thats a tough one. I guess I would just start out by describing what respiration is. "To breath" to the kid. The exchange of gases to us. Then I would say that all things need to "breath" and that includes cells. We breath by inhaling, while a cell breathes by letting in and out certain molecules. Without getting into detail this is pretty hard. You could try to say something about how this works at a cellular level but it may be difficult. Good Luck. cheers!

I'll take a crack at it

Try and explain the concept of cellular compartmentalization in terms of knobs (electron carriers) in a candy machine (cytosol/mitochondria). If the child wants candy (ATP), he/she has to put in a number of different types of coins (glucose;ATP) to turn certain knobs in the candy machine. If the right types of coins are inserted, the right type of knob is turned, and thus putting the child closer to attaining his/her candy.

Of course, the child wouldn't understand what an electron carrier or ATP was, but if they understand the proxy agent, say a 'knob' or a 'candy machine' they would understand the quantitative input necessary to generate the 'candy', or 36-38 ATP molecules that result from each succesive round of cell respiration.

Ask questions to keep them interacting and yeah like someone else said descriptive pictures is a good idea. Make sure they know how it pertains to them.

I wouldn't get into the actual chemical reactions and the electron transport chain and how chlorophyl bounces electrons around.  

Something along the lines of, "The plant uses energy from the sunlight to convert carbon dioxide and water into sugars and oxygen gas.  This is a chemical reaction that uses energy, and the energy is effectively stored in the chemical bonds of the molecules of sugar that it makes.  You could talk about how a plant can go on to string sugars together to make starch or cellulose, if you like.

Later, the plant or an animal that eats the plant can get the energy back out of the sugars by reversing the process that the plant went through to make them.  Instead of using up energy and giving off oxygen, disassembling sugar into water and carbon dioxide gives off energy that the cell can harness to fuel its biological processes, and requires oxygen.

It might be a good idea to have a very simple diagram of the input and output of each process.  Maybe it would be good to talk about how sugar is made out of carbon, oxygen, and hydrogen.

If you want to talk about ATP, you could talk about how using a molecule of glucose to make several molecules of ATP is sort of like going to a change machine with a ten dollar bill and getting quarters that you can then use to buy snacks from a vending machine, run the machines at the laundromat,  use a pay phone, or play arcade games.  (Things that won't usually accept a ten. Although I realize there are high tech exceptions to the rule.)

Edited to add: If I were you, I wouldn't explain the electron carriers.  It's not that an 8 yr old is incapable of comprehending them if they wanted to, but that stuff can put a class of university students to sleep. My personal opinion is that if you try to explain it in that much detail, they will all zone out.  Unless you have explicit instructions that you have to cover the details, go way more general.