If you were to take a plastic bag and place it inside a large bowl, and then, using a wooden spoon, stir the bag around, you could use the expression… - Daniel Handler

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If you were to take a plastic bag and place it inside a large bowl, and then, using a wooden spoon, stir the bag around, you could use the expression 'a mixed bag' to describe what you had in front of you, but you would not be using the expression in the same way I am about to use it now. Although 'a mixed bag' sometimes refers to a plastic bag that has been stirred in a bowl, more often, it is used to describe a situation that has both good parts and bad parts. An afternoon movie theater, for example, would be a mixed bag if your favorite movie were showing, but if you had to eat gravel instead of popcorn. A trip to the zoo would be a very mixed bag if the weather were beautiful, but all the man and woman-eating lions were running around loose.