Were you out sick that day when the teacher explained the menstrual cycle? Or, perhaps, you haven’t been taught yet?

Don’t worry. A new video from the YouTube series ASAPScience will fill you in. It explains the ins and outs of a woman’s monthly cycle–and debunks a popular myth about premenstrual syndrome, or PMS.

“PMS is not the same thing as your period, which is perhaps one of the biggest myths out there,” Mitchell Moffitt, the series’ co-creator, says in the video. “It actually happens seven to 14 days before a girl even shows signs of bleeding.”

Why do mood swings and other symptoms of PMS occur before the bleeding? As Moffitt explains, it’s all about the hormonal shifts that occur during a woman’s cycle.

“Here I am thinking maybe I’m just having a bad day or I’m being crazy because it’s not happening at the same time as my period, but no, it could’ve just been PMS,” YouTube personality Lilly Singh says in a guest appearance in the video.

Now she knows better–and so do you.