Make a fist. This is about the size of your heart,"" Simon (Spring in America, p. 232, etc.) begins, and with this simple, concrete image he introduces the wonders of the human heart, circulatory system, and blood to a picture-book audience. Elsewhere, even abstract ideas become comprehensible, e.g., the average human body contains about twenty-five trillion red blood cells, or ""hundreds of times more blood cells than there are stars in the Milky Way galaxy."" Stunning full-color photos appear on every page, many taken inside the human body with scanners, X rays, and other devices, and then computer-enhanced. The same science savvy and enthusiasm that has made Simon's titles on the universe so popular has been turned inward to uncover extraordinary facts about the human body.