This question lead to a great conversation about the calcium cycle. Calcium is an element (like hydrogen, or oxygen): there's a certain amount of calcium on the planet--you can't really "make" new calcium, but animals can sequester it in different ways. You might know that one of the reasons kids are encouraged to drink their milk is because it contains calcium--which is the element that strengthens bones, especially for growing kids. Bones are a lot like shells--though our bones are on the inside of our body, they're made of similar stuff and serve a similar purpose--they provide "structural support" (which just means they keep us from being blobs:). While people get our calcium from our food, crabs and other animals with shells sequester little particles floating in the water column to build their shells with. Once a crab dies (or molts), the shell will decompose, releasing little calcium particles into the water to be sequestered by some other animal. The kids were fascinated with this cycle--"that's just like recycling!" one of them exclaimed. "I think the crabs are better at it then we are," observed one of the campers, gesturing at a plastic cup left on the beach. "I'm not gonna get beaten by a crab!" her friend replied, snatching the trash off the beach. Walking back, we talked about how different families and neighborhhods practice recyling, and the kids were eager to brainstorm ways to do an even better job--talk about learning from the environment!
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