there's a kewl movie, 'Frequency', about that. this kid's dad died in a fire when he was really young. one night during a lightening storm, while checking out his dad's old ham radio, he ends up talking to his dad like 25 yrs earlier. he warns his dad about the fire, and his dad lives. but then in the present, his mom dies. turns out the night his dad died, his mom was working at the hospital and when she took the phone call about her husband, her patient was given an incorrect dose of medicine, and died...the patient? a serial killer. when dad survived the fire, he showed up at the hospital, right after mom stopped the incorrect dose from being administered. the serial killer survives and continues to kill more nurses... i know it's a bit wordy, but you get the gist, u shouldn't try to change the past. u just might screw it up.