The way you would put it to a nine-year-old is to say, "If a meteorite was coming to hit the Earth, how would you figure out if it was going to or not?" And if he says, "Well, what? how?" you say, "There's a magic word.
La forma de decírselo a un niño de nueve años es: "Si un meteorito fuera a chocar con la Tierra, ¿cómo saber si chocará o no?" Y si él dice: "Bueno, ¿qué?, ¿cómo?" Uds. dicen: "Hay una palabra mágica.