Discuss how an observer on the ground will show show a different aging process than his twin traveling through the galaxy. Introduce the number of months older the observer will be compared to the traveler upon landing.
Compare the aging difference to 50% of light speed, 87% of light speed, and 99.5% of light speed. Consider what occurs when a space vehicle is hovering at rest and what the difference becomes when motion is involved. Indicate that the speed of light remains constant regardless of motion.