Argh!!

JJ has decided that naptime is not for him today. I put all three kids down about an hour ago, and I keep hearing his little voice. Then I heard some rattling. And some shrieks. So I went in there and put him back down and told him in my sternest (is that the right way to say that?) mommy-voice that he was to lay down and go to sleep. About two minutes later, I hear some giggling and more rattling. So I open the door to see him leap down onto the pillow like he'd been that way all along. Isn't it supposed to be another year or so before he starts pretending like he wasn't doing what I know he's doing? Why does he dive into his bed some days and others, like today, he just dances and sings and plays while his brother just dreams sweet dreams, completely oblivious to my baby boy's hijinx? Can someone please tell me, please?!