Conjunctions are words that link linguistic units such as words, phrases or clauses.
We distinguish coordinating conjunctions such as and, or and but from subordinating conjunctions such as because, since, when, while, etc.
Examples of coordinating conjunctions are:
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Look at this sentence:
Do you think the speaker wants sandwiches filled with cheese and tomato or some cheese, and sandwiches with a tomato filling?
Native speakers probably know what cheese and tomato sandwiches are, but they don't realise that the phrase is actually ambiguous (has more than one meaning).