The Hint is in the short anecdote below

When my son was in 4th grade he had a teacher who liked to bike to school.

One day as he was on his way some dogs began to chase him and he sped up to try to elude them.

Unfortunately his attention to the dogs left him inattentive to the large mirror on a parked truck and as he hit the ground he broke his shoulder blade in several places. That kept him out of school for a few days.

He also broke his helmet.