Es war einmal: So fangen Märchen an.

What does the "So" mean in this.

I get that "Es war einmal" is "once upon a time"
"Anfangen" means start, Märchen means fairy tale
but what's the purpose of "SO?"

Question

Es war einmal: So fangen Märchen an.

What does the "So" mean in this.

I get that "Es war einmal" is "once upon a time" 
"Anfangen" means start, Märchen means fairy tale
but what's the purpose of "SO?"

Answer

The whole translation would be
once upon a time: this is how fairy tales begin