No is Funny

Owen doesn't quite understand NO yet. A very common expression in our house lately is, "No, that's not for babies". But of course, that doesn't stop him from anything. Just like the photo above, he likes to get into this cabinet that he shouldn't and he gets a big smile on his face as I tell him No. I've tried to get stern with him when I say No. I put on my most serious face and say, "No!" and all it does it make Owen laugh. He thinks it's funny when I say No.