Walter Fredrick Morrison, whose post-World War II invention of a 'flying' plastic disc became the American recreational icon known as the Frisbee, has died at age 90 of age-related causes. Wham-O Inc. has sold more than 200 million Frisbees since Morrison sold the company the rights to what he called the Pluto Platter in 1957. The roots of today's aerodynamic Frisbees go back to 1937, when Morrison and his future wife tossed a large popcorn can lid back and forth for fun during a Thanksgiving party.