As mentioned, the app works by taking your temperature. You do need a thermometer and it needs to be a proper thermometer – a two decimal one – you can’t just nick your mum's cheapo one from the odds and ends drawer. When you wake up, before you sit up, you’ll need to take a reading in order to get your “basal temperature”. In the first part of your menstrual cycle, your body temperature is a little lower, then, once ovulation has occurred, your temperature rises due to the increase of progesterone hormone levels. So the app analyses the data you put in, and figures out whether or not it’s safe for you to have unprotected sex.* As the app gets to know your body better, the more green days you will get.