I noticed two people pointing into my rather unusual front yard garden, and so I went out to say hi and see if they had any questions. They were an old white guy and a quiet minority girl. They wanted to know why I had tags on the flower stalks of my daylilies, and the man had guessed they were there to repel birds. I explained that I hybridized daylilies, and the tags had two purposes: to describe seedlings that I wanted to keep and to label seed pods with the parentage.
The man asked a question with the word "designer" in it, and immediately my Jehovah's Witness detector went off! I told them I was an atheist. He asked how I came to not believe in god, and I replied that it was a silly idea. He then made the argument from design (watchmaker argument), and among other things asked the girl if she thought my house had spontaneously appeared or if it was designed. I pointed out that spontaneous appearance was EXACTLY his theory of creation, and that lots of things are NOT designed but develop out of natural processes.
Now, I've been arguing with JWs and other religious twits for more than 40 years, but I don't keep in practice. Even though I read Pharyngula religiously. Here's what I should try the next time:
I would address the girl. "Let me predict how this discussion will go. This man is trying to teach you ancient lies, and teach you how to spread them to other people. He will try to bully me with arguments that are hundreds of years old, and hope that I don't know how to defend myself against them. When he discovers that I know answers to his arguments and he cannot bully me, like all bullies he is a coward and will try to run away so that he will not look stupid in front of you. Every time I answer open of his arguments, he will ignore the answer and switch to another argument: watch him do it."
Then, I should take the initiative away from him. "Who designed YOU?" If he says god did, point out that he was not created, his parents produced him. His parents didn't design him either: they randomly combined gametes. His parents didn't create him, he developed from the zygote.
At this point, he will try to get away (often promising to return later), and I would point out to the girl: "Look! His bullying has failed, and he is trying to get away before he looks even worse in front of you. He can't get biology that we can see with our own eyes right. Watch him: he will never be able to get those answers right. What he is NOT telling you is that his lies have been answered for centuries: you can read those answers in books that he won't tell you about." If he has said he will come back, point out that even if he isn't lying about coming back, he will not come back with the girl because he doesn't want her to learn the answers to his lies.
If he hasn't scuttled away, I would ask him "What is the name of your God who you think did the design? How do you know it wasn't Brahma?" I would turn to the girl and explain that other religions have other creator gods and other sacred texts that are supposed to be true. I would point out that his denial of those gods was a kind of atheism, and that I just denied one more god than he did. But not for the same reason: he makes an exception for his one god to be true. I treat them all the same, all false."
I would want to conclude with something like: "While you are young and haven't read the books they don't want you to read, they try to use bad arguments to trick you into believing the same things they do. They are mean and will reject you if you decide on your own beliefs. Learn about atheism as you grow up." But this is a hard thing to say to an adolescent. JW's really do shun even their own children is they step out of line. I hate that sort of cruelty, and I'm not decided about whether to present that plainly to an adolescent whose development could be really screwed up by parental rejection. There might be less harm if the independent thoughts come later, after economic independence.
My actual discussion was much less organized, and insufficiently aimed at the AUDIENCE: the girl. If I had been smart, I would have pointed out how entirely wrong he was about my labels, and how wrong he is about many other things. I did stop his first attempt at escape. But I only run into JW's going door-to-door every few years.
An even better strategy might be to address only the girl, and say things like "is that what they tell you?" When she would not be able to respond to something, say "I don't expect you to know everything: let's see if this guy knows an answer" and then embarrass him. The goal is to let the girl see what a prat the older person is. The girl is the victim: I won't be able to change the mind of the adult.