I was about six, running in the alley (like the genius I was). Fucking tripped and scraped my knee along the rocks (this is an alley, filled with rocks, mind you.) My knee was covered in blood by the time I limped home. I had the scars until I was about 12.
only thing bad was when i was 10ish, when i first began riding/learning a push bike, 5 times i feel first on my knees on concrete, blood always was seen all over my knees, i was skinny boy for that age, well im skinny now, and my mum thought i would have fucked up knees for the rest of my life or not be able to walk properly or stand up straight, after the 5th time i did it, ever since then i have never fallen off my bike no matter how stupid i would be or if i try to purposely fall it wont let me some how. and its been 9 almost 10 years since.
If you want to fall of your bike just simply stick your foot into the front wheel and it will pull you off your bike and make it topple over you.
Or you can cover the pedals in mud and bike really fast while standing up.
You'll slip and die (happened to me today X_X )