Things I find amusing about SI units: The mole was originally defined by Avagadro's number—~6×10²³ items. We tried to redefine it based on something physical—the number of atoms in 12 grams of carbon-12. We have now come back to our senses and once again define it by it's original definition, just more precisely. No other base unit is like this. They have new fancy definitions. But the mole is as it's always been: Avagadro's number.