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?"
要是面对一个九岁大的孩子，你该这么说： “如果一颗流星向地球撞过来， 你怎么知道撞得到还是撞不到？”