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MB3 140a: Separating Food from Food on Shabbos (319:3 part 1)
When you have a mixture of 2 things you want (neither is פסולת), the one you want right now is viewed as אוכל and the other is considered פסולת (waste). Separating 2 kinds of fish, etc. Separating by size is not considered בורר.
Picking out onions from a salad to give to a friend. Shaking seeds out of a melon.