If they were a well-rested child, I'd tell them to stop. If they were not well-rested, I'd take them for a nap.
Adult? Pretty much ignore it. I did once snap at my sister, lol. She knows I don't enjoy listening to people complain about heat and cold...yet I heard that it was hot, she was hot and "Oh, my God, it's too hot" over and over again. After hearing this hundreds of times for several days (and trying to talk her out of a summer trip before we took it, lol) and saying Yes, it is, I know, and I'm sorry...or just ignoring it...I kind of lost it.
"Yes, it is hot. I know it's hot. Everyone knows its hot. I'm hot, too! See all these people?! They're all hot, too! So you can stop telling everyone how hot it is! We know!"
We continued walking a few paces and she said, in humbly sarcastic tone..."I didn't tell 'all these people.' Just you." I had to laugh.
I apologized later and apologized about 1000x after that. She LOVES to tell people that story, because I so rarely snap at anyone. People look surprised and they always look at me for verification because they can't believe it. I have to say, "It's true..." and I have to apologize AGAIN.
I've spent much more time apologizing than I did snapping. It will never be enough, lol.
Moral of the story: Keep your cool. Don't snap. You'll end up regretting it.
...and don't go to Disney with my sister in the summertime.
