John Lithgow is embarking on a singing career. The star of “3rd Rock From the Sun” is releasing his first album next month, “Singin’ in the Bathtub.” The collection of swing-style tunes will be aimed at children. The actor said singing to kids is natural for him. “Back in the early ’70s when my first son was born, I taught myself to play guitar because I loved the idea of playing and singing for my own baby,” he said. “Ever since then I’ve always sung for kids in classrooms, at benefits and at music festivals.” Lithgow even wrote one of the songs for the album.