Here's why:

* Milliliter (mL) and Liter (L) are units of volume. A milliliter is one thousandth of a liter (1 mL = 0.001 L).

* Gram (g) and Kilogram (kg) are units of mass. A gram is one thousandth of a kilogram (1 g = 0.001 kg).

They have the same relationship: a smaller unit (milliliter or gram) is one thousandth of a larger unit (liter or kilogram).