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Name: Leslee
Status: educator
Grade: 6-8
Location:  OK
Country: N/A
Date: March 2007

Question:
I am a 7th grade science teacher and need a good lab activity to go with the excretory system section of our studies...any ideas? Please consider that we don't have any high tech lab equipment....



Replies:
Leslee,

One idea for an excretory activity demonstrates how the kidneys work as filters. You can make it as complicated or as simple as you like to be level-appropriate for your students.

The basic idea is to get some cheesecloth, or some other fine-mesh fabric (if you can get actual filter paper, that's even better), 1 gal water, food coloring, 1lb fine sand, and a tall, narrow glass jar. Mix the sand and water to form a thin, loose mixture, and add some color to it. Fill the jar half-way with just water, and drape the cheesecloth over the glass jar (fold over the cheesecloth to make it many layers thick). Pour on the sand and water to fill the rest of the jar, and watch the bottom layer turn color, while none of the sand flows with the color -- the cheesecloth stops it. You can pull up the cheesecloth, pour out the bottom water (=just like urinating), and then add more water to the top. The top layer will gradually lose its color after several changes. This is like the kidneys cleaning out toxins, but retaining blood cells. You can substitute lots of other materials that you have on hand, but the point is to have two easy-to-see components, one that will move through the cheesecloth, and one that won't. For more complicated experiments, you can have the students calculate how much food coloring must be added to maintain the color -- this can be used to describe how the body must maintain a salt balance.

A variation on the same theme demonstrates how the colon removes water from the mix of digestive fluids and food leftovers. Start with the same cheesecloth, some regular (not light) yogurt, a strainer, and a bowl. Be advised: this experiment can't be done in one class period. Basically, you put the cheesecloth in the strainer, put the yogurt in, and place it in the fridge over a bowl to catch water that drains through the cheesecloth. Over a couple days, the water will drain, and the yogurt will harden, like cream cheese. If you compress it with a heavy can on top, it will get even firmer. 1 cup of yogurt will make about a half-cup of yogurt cheese (more or less, based on how much time you leave it in there, and if you compress it). For extra-grossness, you can add food coloring (or cocoa) to make it brown (yuck!!! -- the kids will LOVE it), and the yogurt cheese is actually very delicious -- I like mine with some cherry preserves on some baguette. Add some fresh lemonade, and you have an extra-gross excretory-themed snack! (ewwww!)

Any of these can be scaled up or down to fit your size needs. Hope these help!

Burr



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