I came up with this terrible pun reading the latest Wen Spencer series and reading the history of French post revolutionary measurement.
What do you call a kilogram of crows?
The answer is two words with a total of ten characters.
The answer is
because a kilogram
is a unit of mass,
and, as greenturtle3141 pointed out,
a group of crows is called a "murder" (just as a group of lions is called a "pride", etc.)