What meaning of one gallon of water?

Answer 1

You have a gallon of water. There is no deep philosophical meaning.

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Another contributor was way more technical, but only slightly more philosophical:

"1 gallon" is just another way of saying . . .

128 fluid ounces

231 cubic inches

4 quarts

8 pints

16 cups.

Note that any of these is just a description of an amount of space. It doesn't matter

what's in it. It could even be empty space. It doesn't change a thing. The numbers

and the meanings are still the same.