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That sack weighs 4 stone.

That sack weighs 56 pound.

That sack weighs 56 pounds.

That sack weighs 4 stones.

Yeah, English is weird.

edit: From wikipedia

The stone remains widely used in the UK and Ireland for human body
weight: in those countries people may commonly be said to weigh, e.g.,
"11 stone 4" (11 stones and 4 pounds), rather than "72 kilograms" as
in most of the other countries, or "158 pounds", the conventional way
of expressing the same weight in the US.[38] The correct plural form
of stone in this context is stone (as in, "11 stone" or "12 stone 6
pounds"); in other contexts, the correct plural is stones (as in,
"Please enter your weight in stones and pounds").