. >> we do with that osteosarcoma in children. we cut out the piece of bone that has the cancer. we put in a titanium rod and then every couple of months as the child grow, we have a magnet that makes that rod grow with the child. and years ago, you would amputate the leg. >> amazing. i want to hire you as our chief booker. you've managed to get jennifer aniston, robin williams and others all to do these commercials. i can't get any of them on my show. how are you doing this? >> bathtub is one of the ways. >> jennifer has been with you for a long time. >> some girls wish to grow up and be a princess. other girls wish to simply grow up. may and bailey are friends battling kidney cancer at st. jude children's research hospital. we developed a treatment for their cancer that's helping kids like them across america. >> you know what i wish for? >> what's that, honey? >> my hair back. >> and no more cancer. >> well, girls, st. jude is working on that. >> is it hard to persuade the big stars? >> well, you know, i try to get them down there. the real secret is going to st. jude and seei

. >> reporter: three years ago, zach was diagnosed with osteosarcoma, a rare bone cancer that targetss, endless chemo therapy and mostly bad news. zach's doctor has told him he only has months to live. his mom suggested he start writing good-bye letters to family and friends. >> i am awful at writing. i can't sit down and write a letter. i've tried. i decided to pick up my guitar and started playing music. this is fun. >> reporter: that's how his song called "clouds" was born. a good-bye to those he loves. >> you want to leave a melody behind? >> i do. >> reporter: and you think that will help them cope? >> it's kind of me always being there for them. like if they keep saying that song throughout their whole life, i'll be right next to them the whole way. >> reporter: zack can't stop writing lyrics, so many songs he wants to leave behind. >> you have to live life fast. you can't really put on the brakes at all. i can't wait for anything. and so that's kind of like one of the things when people say hold on, we have to wait for this. i say i can't. let's go. tomorrow, what are we doing?