Instruction:
what is considered the birthplace of mountain biking?

Response:
Fairfax California is considered the birthplace of mountain biking.  Local high school students who grew up in the area would take old single-speed bikes with coaster brakes up local fire roads and then ride them down the hills as fast as they could.  Eventually they started adding gears to the bikes so that they could pedal them up the hill as well.  The most famous trail was named Repack, because often after the races riders would have to re-pack the coaster brakes with grease because the grease would vaporize from the heat from the hard braking as they raced down the hill.  Modern bike brands like Fisher, Ritchey, Klein, Breezer, and Specialized all had their start in the Marin County foothills north of San Francisco, in the small town of Fairfax.