God dammit, I never get these before they have a 5 thousand comments, but...I'm very good at telling when my sister is on her period. I can even predict what day she starts, once or twice to the exact hour.

It all started back in high school when I (stupidly) went "what, are you on your period or something?" When she was being cranky and she flipped out and said, "You know what? From now on I'll tell you when I'm on my period." For the next 4 years she told me every single time, until I graduated and moved off to college. I didn't think anything of it until I looked at the date one day and said, "she started last night" to myself when it hit me, and after a few days of living with this revelation I called her up and asked and yup, I was right.

Another 3 years later and I'm still able to do it, even without her input, even when she syncs up to different cycles with her friends or coworkers. It's like a crazy ESP, it's evolved a bit to that I can predict the periods some of my closer friends and family members.