Knowledge: The small intestine is the longest part of the digestive tract measuring around 20 feet in length in an average adult while the bone marrow is the tissue that undergoes the most rapid growth in the body.

Conversation:
My hair is brown, how about yours?
No hair here, but your hair would be the second most common color for human hair, only behind black hair.
Neat, do you know why people's browns are different? I thought brown was you know.. just brown.
This is due to the difference in Melanin, the pigment responsible for hair color
Ah I see. It's interesting how much we don't know about our hair
Speaking of that, did you know it's the second fastest growing tissue in the body?
Wow! I never knew that. I always thought it was the digestive tract, particularly, the small intestine. What's the first then?

Response: The fastest growing tissue in the body is the bone marrow.