It's funny Alfred because that negative by a negative thing is something I deal with at work (teaching prospective mathematics teachers) and I think you'd find that a quick survey amongst secondary mathematics teachers would also reveal many don't know why this is the case. At least that's what I've found in this part of Australia. I don't see why it's childish - perfectly legitimate question.
Happy to scan an article (or provide references) & some course notes if anyone would like them (hmm, this is getting susupiciously close to work now - may have to send an invoice to Pelican
)
I think questions like that are really good for seeing how many people have a solid grasp of basic concepts & how many are just following a line of thought or procedure they were taught. Mind you, there's also the case where many may simply have forgotten (or not taken in) what they did back in a maths class when they were 12 or so. Many of us in that boat I'd say.
EDIT - I also believe blind faith in anything (including science) can be dangerous. Always good to question things and good scientific practice is based on this principle.