HI. No, a cup of grapes is not 8 ounces. People often confuse weight with volume which are NOT the same. 1 cup, the VOLUME measure of a liquid, will weigh 8 oz if the liquid is water.
One cup, of a solid mass, depending on interstitial spaces (the empty space created by the grapes, or for example, the hollows created by pasta like ziti) will not weigh 8 ounces, so you have to be careful about measuring things.
When a serving says "1 cup" they mean the volume. If you look closely at the books, they often show a volume measure in conjunction with a weight -- Melon, 6 oz or 1 cup.
The way to "play this game" is to take a bunch of stuff, and a liquid and dry measuring cup (the kind you'd use to measure a cup of flour or sugar) and fill them with foods of different densities -- and then weigh them (of course subtracting from the total the weight of your measuring device.) 1 cup of rice cooked and raw will weigh different from each other; 1 cup of sugar weighs twice what a cup of flour weighs; 1 cup of ziti weighs less than one cup of pastina or ditalini. and so on.
So, again, don't mistake weight measurements for volume. One cup of heavy cream weighs less than 1 cup of skim milk -- but they are still one cup of volume....
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