E.R.
All kids go through this. I personally dont believe in hitting. Why do think kids hit other kids?? Because they get hit at home!! Plus you don't want one twin hitting the other since he saw you do it. Kids imitate every thing. Kids have to be taught everything. For me this is what worked. When it happened I picked up the sippy and said "Milk all done" and put it back in the frig. He learned that this wasn't what he wanted so he stopped throwing it. Then I noticed at the end of mealtime he would throw it too...so I would try to get to him before that or pick it up and ask "all done?"....Now he knows to give it to me and voices "done". They will learn!