By night, they're ruthless hunters that stalk their prey with a keen sense of sight and smell. By day, they're playful, loving companions for millions. Today, cats outnumber dogs as America's favorite pet. Worshiped in ancient Egypt and persecuted during medieval times, domestic cats over the centuries have been feared and adored. As comforting companions, cats provide therapy for the elderly and autistic. But as dramatic sequences show, the behavior of cats is never far removed from that of their cousins in the wild. If you've ever wondered why cats always land on all four feet or what makes them purr, watch this video.