I actually took a "how to" course on vaginal secretions once. I know it sounds strange, but it was to learn about your mucus as a barometer of fertility. The mucus/secretions change through the cycle, and you can know your absolute most fertile days by the type of mucus. I learned to be able to tell within a few days what part of the cycle I was in just by looking at and feeling a bit of the mucus. This was very useful in getting pregnant as I was living far away from my partner and we needed to be able to time our visits perfectly to aid in conception. There's this one test where you put a bit of the mucus between your thumb and finger and draw them apart. The most fertile fucus does not break and makes a longish strand between your thumb and finger. Non-fertile mucus tends to not stretch at all or only make a short strand. The mucus helps determine fertility because its character can aid or hinder/block sperm on their journey.