what would you do if a teenage girl...?

You should tell her parents what you know.

You know when I was younger I would have never gotten away with that, because you can bet your a** the neighbor would have been knocking on the door, asking what your doing, and when your parents got home you knew there was going to be trouble..Those were the good ol'days.. Everyone looked out for everyone, Now everyone seems to think you should mind your own business..No matter what.. Tell her parents, she might thank you in a few years..

I would say to somehow tell her parents, if only because, like you mentioned, something could happen that they'd regret later and if her parents don't find out till after...there's nothing they can do about it. I would also, however, think of the reason that you would be telling someone about it. You say "you aren't cool" with the person so...I would ask...are you telling on her because you honestly think something bad is happening or because you want to get her in trouble? Good luck with the decision.

I would probably call her parents or ring the door bell and pretend that you have to tell her something or that you want to talk to her mom because she will most likely answer the door which you will then be able to see if she is doing anything wrong if thats what your concerned about. Maybe when you comfront her mom you could ask her not to tell her it was you and have her mom walk into the house when she is home alone with the boy without her knowing.

Just ignore it its not worth your time in fact you really shouldn't be looking over there to begin with. How would you feel if she was posting this about you and was asking advice on what to do about something that to begin with had nothing to do with her. You would be pretty hurt wouldn't you you ever heard the saying treat people the way you would like to be treated well this is a perfect example of that would you want to be treated that way.

Not sure what "u aren't cool with" means in this context, but I'll assume it means you aren't friends with the girl. If you don't know the girl, I'd say mind your own business.

If you are friends with her parents, you could mention it to them. If you aren't, then I dont thinkit's enough of a potentially-dangerous situation to warrent bringing it to their attention.

I would tell her you saw her. Watch her reaction. if she doesnt care dont bother. if she cares tell her she should stop (and tell her the "regrets she'll have to live with) or else ull tell her parents (or someone in charge of her).

The nighbors don't need to get any funny ideas that I'm going to be their future babysitter.

I would say find out, maybe give a call if you know their number, or ask normally "Who was that guy that one day?"

but before that maybe say "Hi" and start a conversation.

honestly what your neighbours do is none of your business and you should let her make her on decisions if she messes up she'll learn a lesson guaranteed