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World's Roundest Object!
Veritasium

Veritasium explains the effort to redefine the kilogram, which was still defined by a physical artifact, and how a near-perfect silicon sphere from the Avogadro project helps. It also covers the history of the unit's odd naming.
Recommended in kilogram-redefinition threads as a clear, short explainer of the Avogadro and Planck approaches.
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