Riddles in the Dark - Thirty white horses on a red hill...

Riddles in the Dark - Thirty white horses on a red hill... (23rd street station; uptown N & R trains)

Riddles in the Dark - Thirty white horses on a red hill... (28th street station; downtown 6 train)

Riddles in the Dark - Thirty white horses on a red hill... (5th Ave/59th street station; downtown N, Q & R trains)

Riddles in the Dark - Thirty white horses on a red hill... (5th Ave/59th street station; downtown N, Q & R trains)

Riddles in the Dark - Thirty white horses on a red hill... (Bleecker street station; downtown 6 train)

Riddles in the Dark - Thirty white horses on a red hill... (Bleecker street station; uptown 6 train)

Riddles in the Dark - Thirty white horses on a red hill... (59th street station; uptown 1 train)

Riddles in the Dark - Thirty white horses on a red hill... (14th street station; uptown 1, 2 & 3 trains)

Riddles in the Dark - Thirty white horses on a red hill... (Canal street station; uptown 1 train)

Riddles in the Dark - Thirty white horses on a red hill... (Christopher street/Sheridan Sq station; uptown 1 train)

Riddles in the Dark - Thirty white horses on a red hill... (2015)

Answer: Teeth.