This is a writing exercise for the student, and answers will vary.
Work Step by Step
Sample: Dear Grandpa, I know that you are worried about radioactivity, and that you think it is something unnatural and that is relatively new. However, I'm writing to tell you otherwise: radioactivity has been around as long as the Earth has, and is a part of nature, even though we didn't know about it until relatively recently (the beginning of the 20th century). About 75-80% of our annual radiation dosage comes from natural sources, as my textbook says on pages 619-620. We ourselves are radioactive, from the potassium in our bodies, and from ingesting the radioactive Carbon-14 that is continuously being generated in the atmosphere. In fact, one of the ways that we can date ancient objects, known as carbon dating, is by measuring how the relative amount of radioactive Carbon-14 in a formerly living plant/organism has decayed over time. Radioactivity also provides some of the heat that keeps Earth warm, and it heats the water in hot springs.