First and only concussion: Mountain biking for the first time, summer after freshman year of high school (I was 15). I went down a hill and didn't use my breaks enough or lower my seat. It was fun until I hit the rut at the bottom. Front tire went perpendicular and my head, and collarbone hit the handlebars. Fortunately, the bike helmet and collarbone took the brunt of the damage.
My friend said I was totally out of it and couldn't remember where I was. We were only a couple miles from my house, so he had me follow him and we walked the bikes back.
