When a wildfire swept through 2,000 acres in the hills north of San Diego, California, in May, it left behind a curious checkerboard of destruction. One neighborhood was reduced to ashes and twisted metal. But houses nearby suffered little more than scorched grass and singed trees.

Captain Richard Cordova of the state firefighting agency, Cal Fire, visited the scene and quickly hit on one explanation for the fire's apparent favoritism.

Many of the homes that survived had little flammable brush in the area immediately around them. But others had dry grass and shrubs within feet of the walls. "Those were the ones that unfortunately did burn down," Cordova said.

As the summer wildfire season enters full swing in the United States,