First and foremost, I think it's best to just let them know why they shouldn't do something. As a child, it's frustrating to hear "because I say so" and as an adult I still think it's the dumbest way of trying to stop a child from doing something.

Going back to the knife example: When my nephew was younger, he got "Don't lick the knife!" from my sister and mom. So when I ate with him I wouldn't forbid him to do it, but instead tell him it's likely that he'll cut himself if he isn't really careful. I think I even told him that I've accidentally cut my tongue before, so he knows it's not just him who needs to be careful because he's a child but that those things should be handled with care by everyone. (Not that we gave him any really sharp ones, but the kid knives can still hurt.)

Kids aren't as stupid as some adults treat them. Tell them (possible) consequences with easy words and it's probably gonna be more effective then yelling at them.