Chalk is the fossil fuel of modern mathematics. It was formed in the Cretaceous period, roughly 100 million years ago, when the seas swarmed with foraminifera and other planktonic organisms whose calcium-rich skeletons accumulated in thick beds of the soft, white stone. Now the chalk is quarried, refined, and pressed into crayon-size sticks that mathematicians delight in stroking across smooth slate. A chalkboard is the preferred medium of expression for many kinds of mathematical discourse: solitary ruminations, teaching, presenting work to colleagues, collaborative research sessions.
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