Kidneys

Olivia and I were reading one of her favorite Usborne books, "See Inside Your Body" when she asked, "Do grown-ups have kidneys, too?"
I said, "Yes. Everyone has kidneys."
She replied, "Really?!" Then pointing to her knees, she said, "But these are real kid knees!"
I said, "No not kid knees.....kidneys. Wait that does sound the same."
In my 29 years of life, I never put that connection together!