Raw fish is sizzling hot right now. Los Angeles has brand new bars devoted to an Italian style of raw fish, called crudo. President Obama kicked off his visit to Asia last month noshing nigiri at Jiro's famous sushi bar. And back in December, The New York Times named Sushi Nakawaza as its top restaurant of the year. But why do so many of us find utter bliss in eating raw sea creatures but aren't so inclined to chow down on uncooked birds, cows or pigs? A big part of it is gravity — or the effective lack of it...