Jace is really getting good at throwing fits. If he doesn't get his way he is bound to give the worlds greatest frowny face and start this dramatic pouting.
But if you don't give into his antics and you just look at him, he tries to stay mad but breaks out into a mad laugh. You know the kind of laugh when your trying to be mad but you really have nothing worth being mad at? So you are forced to laugh but it makes you even more mad, and mean. Ya that's what happens.
It's actually quite entertaining.