Nuts are their winter food.
Meaning: Squirrels rely on nuts as their primary source of nourishment during the winter when other food sources are scarce.
Choice of Words: Nuts are the hard-shelled seeds of certain trees. Are indicates a state of being or existence. Their refers to squirrels. Winter is the coldest season of the year. Food is anything that nourishes an organism. When you put them together, "Nuts are their winter food" means that nuts are the food that squirrels eat during the winter season.
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