I found this great book at Border's clearance shelf ($5.99). It has the different body systems outlined and a brief description of different parts. It is laminated and comes with a pen so the kids can practice writing the body parts after they read about them.
Little J picked out the heart to read and study. He was able to listen to his heart with a stethoscope. I had him listen first after he had been sitting for a bit and then after he had jumped and ran for a bit. He was able to notice the difference in his heart rate right away. We also felt our pulses and listened at the same time. He found that rather amusing.
We followed up our lesson by watching the Magic School Bus Works Out. It's a cartoon based science program. For the most part it's pretty good, a little goofy at times, but he seems to learn from it. This episode was more related to the respiratory system, but shows how the body's systems work together to make us function.
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