First lady Michelle Obama will deliver the comic Top 10 list on David Letterman's Late Show tonight. The main topic: Gardening. Mrs. Obama is promoting her new book, American Grown: The Story of the White House Kitchen Garden and Gardens Across America. Mrs. Obama last appeared on Letterman's show on CBS in March. Maybe Mrs. Obama will use the Top 10 to take a few jabs at her husband, the president -- or perhaps the family dog Bo, the subject of her last appearance on Letterman. "He is my son," Mrs. Obama said of Bo. "I have two girls and a boy." Mrs. Obama also said Bo has "a very busy schedule. We're thinking about getting him an agent."