"TIL that when you get a kidney transplant, they don't replace your kidney(s), they just stick a third one in there"

From the Mayo Clinic:

The surgeon makes an incision and places the new kidney in your lower abdomen. Unless your own kidneys are causing complications such as high blood pressure, kidney stones, pain or infection, they are left in place.

And one of the top comments:

It’s true! My dad currently has 4 kidneys