In third grade I got a new bike.
I was a careless rapscallion so I rode it to school as fast as I could. When the day was over, I was so excited to ride it home.
I cut across this sort of dirt path that intersects with a street. Coming out of the dirt path, I had to cross the street, but instead of stopping and looking both ways like a good citizen, I just sped across at high speed (maybe 20 km/h) and ended up looking down the street (backwards) while crossing it. When I turned back around I was unconscious.
What happened was the front wheel of my bike clipped the corner of a parked car, and the rubber on the tire stopped, but the rest of the bike swung around nearly 360º and my face struck the back windshield and cracked it. I was knocked unconscious, broke my jaw in three places and shattered four teeth.
I lay unconscious in a small pool of my own blood until my friend Melissa came out of the dirt path, screamed, and ran to get help.
Two of the teeth were permanent, so right now I have a Marrion implant where my two front teeth should be. It looks completely real, but when I turn eighteen I have to get permanent implants drilled into my jawbone.
I had to go into surgery to get the teeth pulled out and had to have my mouth sewn up. I spent a week in the hospital and paid for my mistake with years and years of in-mouth retainers and trips to the orthodontist.
Look both ways.
And wear a helmet. It saved my life.