For a young parent, having a two year old stand in front of you and say "NO" in an emphatic tone is a challenge to their very souls. This is where I come in to talk about "developmentally appropriate behavior". Of course, the first few times the parents don't want to hear it; they want, 1. for me to acknowledge that their child is really Satan in disguise, 2. for me to say that it is all right to beat the child, or yell at them, or shake them, or use some other method that scares the sh.. right out of them.
And I won't do any of these things, so now they are angry at me. Until...
I tell them that they are good people and good parents. "Look at all you do for your child," I say. "You have fed and changed and rocked and loved this baby, and BECAUSE YOU HAVE DONE ALL THESE THINGS, you now have a healthy, happy child that has a healthy self-image and is going to be a strong person. I know it's tough, and it's daily, and now you have to let go a little, but you've done a great job, and that's why you are at this point with this child." Sometimes it takes four or five weeks, with the help of some techniques, to get them to relax and enjoy the little beast.
Anyway, that's two year olds. And, I might add, gently, that it's also teens, who go through the same developmental stage and have the same needs as a two year old. (Don't breath a word of that, any teen worth her salt would be highly insulted.)