Make two fists and place them together so your knuckles are touching each other. Slide one hand toward you and the other hand away from you so your knuckles rub together. What happens? What might happen to two tectonic plates as they move this way?

The knuckles are like a jagged surface. When they rub together, they act as obstacles to each other. It’s difficult to move the hands freely as the knuckles catch and release one another. In the case of tectonic plates, if two plates rub, they release large amounts of energy, which can cause earthquakes.

the corpus luteum secretes estrogens and progesterone. the latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. if the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.

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