dealing with tempers.....

ok, I need to really get out the books for coping with a childs temper. Is it normal for a 9 month old to have a temper so strong? Even my sister (her full time babysitter) said she is really strong headed when it comes to showing her temper. She tends to hit back at me if I tell her no. Such as.... tonight she was sitting in the kitchen sink and playing with the water. I told her to not play with the handle so that she does not turn it to HOT water. I know, she does not really understand that yet, but I know she knows the word NO. I said Julia, no, don't touch the handle. She immediately pushed my hand aside each time and then slapped me with her hand. Intentional slap too! How do I curtail this slapping? I know she will go through stages of different levels of her temper, but how do I get the message of no through to her without her hitting back? She slapped my niece (who my sister also babysits full time) the other day while they were both in the kiddie pool. Julia wanted the water hose to herself and apparently slapped my niece with her open hand when my niece tried to reach for the hose too. Bahahahaha

I have to laugh at this, but her temper is so strong. Any suggestions?