My 10-year old son asks: "If the world started with Adam & Eve, and they had 2 sons, Cain and Abel, and Cain slew Abel -- then who was Cain afraid would slay him as he wandered the earth?" Well, ya got me, I answered. 

Then he asked how there could be any others if the lineage ended with Cain. Well, ya got me again.

I'm feeling fairly ignorant about this very simple story. "Why is there air?" would have been much easier. I humbly ask for guidance.

You have quite a clever son! 

Cain was afraid that God will punish him, and one of his descendants will kill him. Alternatively, he was afraid that the animals will kill him. (source: "Rashi" Genesis 4:15) 

Why is there air? I believe Bill Cosby has a CD on that topic.