The world's roundest object helps solve the longest running problem in measurement -- how to define the kilogram. Support Veritasium on Patreon: http://bit.ly/VePatreon A kilogram isn't what it used to be. Literally. The original name for it was the 'grave', proposed in 1793 but it fell victim to the French Revolution like its creator, Lavoisier. So begins the tale of the most unusual SI unit. Th