What are some objects in a classroom that contain calcium?

What are some objects in a classroom that contain calcium?
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You can find calcium anywhere in the classroom. Chalk is composed of calcium sulfate. Calcium oxide and calcium phosphate are used in the making of glass, which is found in classroom windows. Calcium carbonate and calcium stearate are used in the production of cement. Calcium tungstate is used in paint and light bulbs, and calcium and its compound variants (hydroxylapite) keep the kids alive and healthy.