What could happen if you don't wear a bike helmet?

A wide range of things can happen if you don't wear a helmet

• maybe you never fall hard enough to hit your head, in which case nothing happens.
• maybe you take a sorta bad fall, scrape or split your scalp open. It'll be a day at the ER getting the wound cleaned and closed.
• maybe you fall a Little harder, get a concussion. Can mean anything from a passing headache to life long migraines
• maybe you get a fairly nasty brain injury. Migraines, memory loss, personality Changes, epilepsia...
• Death
• there's the law too. some places require (some) riders to wear a helmet. If you don't, and get caught, you will be fined.