In your knee there is a small priest named Kneecap.
This is a really common, cute way of talking about your kneecap. It's usually used by kids and adults alike, and it's more common in spoken Japanese than 膝頭. Kind of like how we use "kneecap" instead of naming the "patella" bone all the time.
This word uses the kun'yomi readings for 膝 and 小, but the on'yomi reading for 僧, which gets rendaku'd from そう to ぞう, so watch out for that! You've learned all these reading before, so you should be able to read this as well.
I fell off my bike and my kneecap hurts.
I recently got a tattoo on my right kneecap.
How's your kneecap?