Nipple or pimple?

My kids do this new annoying thing. But they wait long enough in between doing it - that I forget, and they get me again with it.

Zach started it. Now they all do it to me at random times.

Zach: Mom. Mom. Mom. Mom. Mom. Mom. Mom. Mom. Mom.

He'll keep going like that until I say...

Me: What?!

The he takes his right hand, and lifts up the corner of his t-shirt super fast to show his nipple and says:

Zach: Nipple.

And then quickly pulls the shirt back down and walks away.

One evening, when his friend Carter was over - he kept doing this to me.

Finally I said...

Me: Do it one more time Zach... and I'm going to do it back to you.

Carter: Do it Zach! She won't do it!

Zach: Uhh... you don't know my mom very well.

Me: Watch me.

pause

Zach: Mom. Mom. Mom. Mom. Mom.

to which I responded...

Me: Zach. Zach. Zach. Zach. Zach. Zach.

while holding the corner of my t-shirt... waiting for him to say "what."

I think it was the holding of the t-shirt that scared the two of them. They ran away.

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Ethan does this to me too.

But for some reason, instead of saying "nipple," he lifts up his shirt and says "pimple."