LAS VEGAS — The other day my girls put on princess dresses and paraded around the house.

"I look pretty," my 5-year-old told her younger sister. "And you look disgusting."

I was so shocked at the comment that she immediately was sent into a time-out while I tried to figure out why she said those nasty words.

My husband and I wanted to know where this behavior was coming from. I assumed some of it was natural, some of it was jealousy of her sister, but some of it had to be coming from a kid at school, right? Who was to blame?

Other phrases have also recently popped up: "You're not my friend anymore," "Why should I?" and "Who cares?"

Immediately my husband started quizzing the 5-year-old. Who talks like this? Did someone treat you this way? The 5-year-old seemed to make up names as the interrogation continued and couldn't explain the behavior at all.

What happened here?

I want my 4-year-old back, before she entered real school and never said cross words.

Gary Direnfeld, a social worker in Canada and expert in child development, said we are going about this all wrong.

"It typically isn't a good strategy to ask a child where they came up with something or why they said what they said," he said. "When we pressure them to provide a rationale, they do their best to simply come up with whatever is convenient to appease us. Typically, they're looking for the expression on our face to know if they've come up with something."

That did seem to be what happened during our questioning.

My daughter is going through a development shift, he said. He agreed that some of the behavior might be natural and a little jealousy over a younger sister, but he said parents shouldn't always assume their child is picking up things from another child.

"It might be your child who's the instigator," he said.

Ouch. Never thought about that.

He did say some of the phrases we've been hearing around our house are part of kids discovering who they are and trying to assert their separateness from others. And, of course, the old "monkey see, monkey do" pattern plays a role. If this was done to my child, then she thinks she gets to do it to someone else.

So what I need to do, he said, is not give a lot of admonishment, but explain to my daughter by direct instruction and then redirect the behavior.

He suggested saying something like: "You may feel that way, but it's not acceptable to say and it's not acceptable to act on that."

At this age, my daughter doesn't necessarily know how to discriminate the good from the bad. It's up to us as the parents to guide that.

"By the time your kids are teenagers, it's hard to be a director," Direnfeld said. "You're a consultant."

We stopped the interrogations, and when our daughter does blurt out nasty comments to her sister or acts inappropriately, we have stepped up the discipline. She seems to get that the behavior is wrong.

I just can't help thinking, what will first grade bring? I have to get control of this now. The teenage years are right around the corner, right?