The T word:  "Trespass".  Matthew got hurt sledding on private property.  does he have a claim against the property owner?  You might be surprised...
Another "A" word:  "Assault" (as in civil assault, not criminal):  We hear about it all the time.  But can there be an assault if there was no actual physical contact between the parties?  Another example where the law is different from the common perception...