A very good reason is that postal rates are based on their mass - measured in grams. You can measure the mass in widgets and convert to the units used by the postal service, if you so wish.
gram is the unit for mass, out of your choices.
milliNewtons. A gram or kilogram is a measure of mass, not weight.
use a triple beam balance
The weight unit to use is gram (gramme).
Gram, g
A gram.
paper clips do not weigh much, perhaps a gram each, so 10 weigh about 10 grams, so you would use grams, not kilograms, since 1 kg = 1000 grams
gram (g)
gram
You would use Kilograms to weigh a cat.
Milligram: to measure its mass.
Weigh? mass in kilograms or pounds.