You slumped into Donnies lab, feeling like garbage, and told him your period started.

"Ah." He nods. "I see." He pauses. "Did you know that it's because your body has decided you're not pregnant? As a response, it's shedding the inner lining it's been building up for the whole month, which is released through the vagina. Hormones like estrogen and progesterone regulate these phases and prepare the body for a potential pregnancy. An egg is released from the ovaries in a process called ovulation. The released egg travels through the fallopian tube towards the uterus. If the egg is not fertilized by a sperm, the lining of the uterus that thickened in preparation for a possible pregnancy, thus-" He continues rambling, his scientific brain almost excited to share.