A blackboard (also known as a chalkboard) is a reusable writing surface on which text or drawings are made with sticks of calcium sulfate or calcium carbonate, known, when used for this purpose, as chalk. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone. Modern versions are often green because the colour is considered easier on the eyes.

https://en.wikipedia.org/wiki/Blackboard

"There are all kinds of interesting questions that come from a knowledge of science, which only adds to the excitement and mystery and awe of a flower."
Richard Feynman
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