MELTDOWN

What is it that flips that switch in kids that they have complete and total meltdown? And to make it worse over NOTHING. Yesterday was conference day with Zachs teacher. All went fine, a few backwards letter and he was defined as the class clown by his teacher, but other wise good. We finsih up, get zach and Kev takes him to Mcdonalds (I was getting katie from a friends and was going to meet him there). Zach declared 4 (yes four) times he wanted to just go home and have waffles so he could play with his hackey sack from school. Kev takes him home and he melts down crying and screaming that he wanted mcdonalds. This went on the whole way home, he stayed in the car (in the garage) at home, then came in and went to his room and stommped, slammed and screamed for another hour (during which I tried everything from sitting with him, talking to him, yelling at him...) until I had enough told him to put his PJ's on and go to bed. Kev just sat there and finally he fell asleep at 6:45. He slept all night long too. I was expecting him to wake at midnight starving. He woke up today, didnt seem hungry and never mentioned the whole epoisode and was happy as can be. What it the world caused that??????????????