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I've searched the internet for core exercises for building abs, and I'm a little bit confused. Some say that sit-ups is great for getting well defined abs, while some say you only get better at doing sit-ups, and nothing else. I understand that doing sit-ups is better than nothing, and that it's building up the abs in some way. My question is what is the best exercise for getting as big/well defined abs as possible? And how many reps and sets?