What was Mark Twain's actual name?
Samuel Langhorne Clemens was known the pen name Mark Twain, a nom de plume he coined to reflect his time as a river boat pilot.  On the Mississippi in the 1800's it was common to measure river depth in terms of 6 foot increments demarcated on a rope by knots.  Thus, when the river was of depth 12 feet, or two fathoms, the boatmen would call out, "Mark Twain" indicating the second of two knots was submerged.
Instruction: What was Mark Twain's actual name?
Context: 
Response: Samuel Langhorne Clemens was known the pen name Mark Twain, a nom de plume he coined to reflect his time as a river boat pilot.  On the Mississippi in the 1800's it was common to measure river depth in terms of 6 foot increments demarcated on a rope by knots.  Thus, when the river was of depth 12 feet, or two fathoms, the boatmen would call out, "Mark Twain" indicating the second of two knots was submerged.