I get asked this frequently by my more advanced students. Basically, an idiom is a combination of words whose meaning is different than the literal meaning of the words put together and a metaphor says that one thing is another thing to compare two things or as symbolism.
These two pictures represent the metaphor “to be the black sheep in the family”. It implies that you have family members who are sheep and you happen to be the black one! Obviously, nobody has a family member that is a sheep but it implies that you are somehow different than your family. For example, I am the black sheep in my family. I am the only vegetarian, the only one who loves to travel, the only one to speak a foreign language and the only one who has ever lived abroad (besides my grandparents about 50 years ago when my grandpa was in the US Army!)
the word sheep is singular and plural. I have 1 sheep, my neighbor has 3 sheep.
This picture represents an idiom. “It’s raining cats and dogs”. Obviously it cannot rain cats and dogs. It’s impossible. But when we say, it’s raining cats and dogs, we mean that it’s raining very heavily.
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