Why is marijuana the 'Gateway Drug'?

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I have never understood how marijuana got this reputation for being the Gateway Drug and I was hoping someone on here could clear this up. I have never heard of any real science behind it but then again I have never really went looking for it. I just never heard of people getting cracked for weed possession and then being found with all these other drugs, but thats a vague example. If I were to define a gateway drug I would certainly say it was Alcohol