<human>: When i was about 11 i was on a all day biking trip with school about 
(at least a few tens of kilometres on a bike trail) 
and on the way back i decided i was going to try and stand on the frame of the bike up straight as a stunt surprisingly IT WORKED! it was amazing but when i tried again i lacked the proper momentum and ran off the trail down a steep hill ~20m i did several flips and finally landed on my back near the bank of a river and quickly recovered the bike and ran up the hill jumped into the trail with my hands in the air an yelled I'M OKAY! with only some mud and a maybe a scratch or two and continued biking, then after wining a race against someone i tried to brake only to find the brake cable on the bike had broken during my fall down the hill and i was unable to stop whilst going very fast down a bike trail (fast enough to crash into traffic or not make a sharp turn)
i guess the fall caught up with me as i skidded on a patch of gravel and cut my knee up pretty bad and still have a scar.

Can you condense the text into three sentences or less?
<bot>: tried doing bike stun fell down ~20m hill uninjured only to find post race that the brake cable had been cut, skidded in gravel, messed up my knee