why u getting asked, u must be saying someing for them to want to know, just agree a price give a business flyer with contract details on and move on to next house, plan on being there less than 2 mins
if they still want to know, i say
do you know when u wash ur dishes the plates shine, that stuff that cleans plates attracts dirt and thats why a lot of trad wc say u need ur windows cleaned ever week or 2, but this is pure water there is nothing in the water to attract dirt so no dirt sticks to windows, then on first clean, clean like hell and rinse loads,
if they are still wondering just say
tell you what, i will clean your windows and show you the results, as seeing is believieng