Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

If the styrofoam is compressed, it still doesn't prove that a helmet
had a protective effect. This can be demonstrated with a fist and a
brick wall.

If you 'shadow box' at the wall but carefully stop your fist about 50
mm before it reaches the wall (be sure it's limited by your arm's
length), no harm will come to your fist. If, without changing your
position, you slip a 75 mm thick piece of styrofoam against the wall
and repeat the punch, you'll get compressed (and cracked) styrofoam
and false 'evidence' that it saved you from harm. In other words, many
impacts of helmets would be near misses with bare heads.

In high impact crashes, such as most that involve motor vehicles or
fixed objects like concrete barriers and lamp posts, the forces can be
so great that a helmet will compress and break in around 1/1000th of a
second. The absorption of the initial forces during this very short
period of time is unlikely to make a sufficient difference to the
likelihood of serious injury or death. It is for this reason that
helmets contain stickers noting that no helmet can prevent all head
injuries.

Ibid.