This may sound like a really dumb question, but how do OPK's work? I've been using 3 apps, and ALL 3 have different ovulation days. So I figured Instead if relying on apps to calculate my ovulation, I'll just do OPK's to pin point it. However, I am so clueless as to how it works. I've been reading posts on here and they have just confused me more. Like for example, a user posted a pic of two tests, one was really dark and in the other pic the line was lighter, I would assume the darker line meant ovulation but according to the responses, the poster did not ovulate yet. So how do they work?