One day I was at some sort of water park with my cousin, who's a toddler, and my aunt. I was sitting down with them. We were watching this kid trying to climb up the slide, but he kept slipping, since it was a dang water park. Again, WATER was there. It was silent until my cousin said:

That kid is stupid.

My aunt seemed shocked for a second. Then she put on her,"I'm-going-to-scold-you-young-man" face, and said:

Don't say that! That's not a nice word.

My cousin didn't seem to mind her disappointment, and said:

You say all the no-no words when you are yelling at daddy. How come I can't say one?

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I want to remind you guys that I'm using the correct grammar when kids are talking because most people I know like the translation there already, and not at the end. And yes, I have actually experienced this, but it's either been recent or a few years ago so my memories might not be accurate.