Apple Idioms:

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apple of (someone`s) eye

- someone or something that one likes a lot or likes more than others

The little girl is the apple of her grandfather`s eye.
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as easy as apple pie

- very easy

The test that I wrote yesterday was as easy as apple pie.
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bad apple

- a bad person

The boy is a bad apple and he is always in some kind of trouble.
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compare apples and oranges

- to compare two things that are not similar and should not be compared

It was like comparing apples and oranges when we compared our new boss to our old boss.
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polish the apple

- to flatter someone

Nobody likes the girl because she is always polishing the apple with her teacher.
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rotten apple

- a bad person

Sometimes there is one person who is a rotten apple in a group of people.
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rotten to the core

- to be completely worthless (like a rotten apple)

The political organization was rotten to the core and everybody knew that it must change.
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upset the applecart

- to ruin a plan or event by a surprise or accident

Everything was going well at the company picnic until our boss arrived and upset the applecart.
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http://www.idiomconnection.com/food.html#A1