if something goes up it must come down

This morning my son came to me with a bottle of that spray silly string. He wanted to do it in the house and of course I nixed that real quick. I did tho let him go outside and do it on the porch and play, with the understanding that if he was going to spray it there were some ground rules. 1. only on the porch 2. can't spray your sister 3. you have to clean up everything when you are done. The fun of the small bottle came and went and keeping it on the porch and not spraying his sister were followed to a T. Cleaning up tho was another story he cleaned up all the big pieces then there were thoes stragglers. My son then proceeded to try to rationalize/argue with me for the next thirty minn. Yep I am that push over mom that if he argues long enough I am probably going to give in cuz I am sick of arguing. I have noticed this pattern with my daughter lately and have really stuck to the time outs. My question this week is do you know what it takes for you to give in to your children? Better yet do your children know what works on you?