Knowledge: Black out curtains are curtains that block the light by one hundred percent. On the other hand, people with black hair are about 50%.

Conversation:
I have brown hair, quite common I guess.
Yes, it is below black hair for most common hair color.
And what is the least common hair color?
Red hair naturally happens in 1 to 2% of the human population although I'm not sure if it is the least common hair color.
Hmm then do you know the percentage for black?

Response: Ah yeah, The percentage for black hair is 50%.