I honestly know that my hobby is so strange in normal people eyes that I don't even try to explain what I'm doing and if someone makes odd comments I completely understand them. But sometimes ..
(I was photographing on street with my DIY 5x7")
Stranger: "Hi, beautiful old camera!"
Me: "Actually this is quite new, under one year old."
Strange: "Uhmm.. Where do you can buy such cameras?"
Me: "Actually I have designed and built this myself"
In this point the stranger is already starting to get confused.
Me: "It shoots this kind of film" (showing 5x7" sheet film)
Stranger: ... (totally puzzled) .. "Do they still sell film?"
Me: "Yes, actually manufacturers are bringing out new products"
In this point the stranger is played out. They usually start walking away and wishing pleasant photographing

If that is not enough, I let them look at under dark cloth. When they come out I usually ask "Do you now understand why you need the dark cloth?" and they are enlightened agreeing in awe.