A recent "chain" email making the rounds warns recipients about so-called "bottle bombs" left in front yards or in mailboxes. According to Snopes.com, the warning is true, and is referring to crude bombs consisting of Drain-o brand drain cleaner and tin foil mixed together inside of a bottle. The chemical reaction between the Drain-o and the tin foil makes a volatile build up of gases and subsequently detonates the bottle with a great amount of force sufficient to sever fingers and deliver 2nd and 3rd degree chemical burns to the unsuspecting victim. The chemicals can also cause blindness (the Snopes.com page linked to above includes a scary video demonstrating the force of these bombs).
A similar warning in the past concerned acid bombs left at doorsteps or in mailboxes.
The perpetrators of these bombs appear to be teenagers bent on mischief. But unfortunately, their prank can have injurious, even deadly consequences.
As a safety precaution, if you find a soda bottle lying around your yard and it shows any signs of swelling or melting, do not touch it. Instead, call 911 to send someone over to investigate. Also, in general, do not pick up soda bottles lying on the side of the road.