This falls into the category of wacky unit conversions,
but is a bona-fide real-world example, not in the same
category as facetious units such as gigaslugs per parsec
cubed or furlongs per fortnight.
It is a relatively advanced example, because it is
nonlinear.
More familiar nonlinear measurement schemes include
logarithmic ones: dB, pH, Richter number, et cetera.
In the category of fun/cool units, I've always been
amused by the abbreviation for the fermi unit:
1 fermi (fm) = 1 femtometer (fm)
On a related note, a facetious non-real-world
conversion would be to compare agriculture to
high-energy physics by calculating the area of
the proverbial broad side of a barn, measured in
barns. One barn = 100 fm^2.
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Here's an addition to the list of useful rules of
thumb: The US "nickel" coin weighs 5 grams, very
precisely, by design. It is also approximately
2 mm thick. This makes it a handy reference.
I made up a mnemonic for this: 5 cents, 5 grams,
and 5 to the centimeter.
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In the category of totally facetious units, we have
things like the anti-millihelen, i.e. a face ugly
enough to sink a battleship.