Answer:
What force and strength cannot get through. I, with a gentle touch, can do. Many in the street would stand. Were I not a friend at hand.
Without a bridle, or a saddle, across a thing I ride a-straddle. And those I ride, by help of me, though almost blind, are made to see.
Always wax, yet always wane: I melt, succumbed to the flame. Lighting darkness, with fate unblest, I soon devolve to shapeless mess.
Neither bought nor sold but more valuable than gold. It is built but not by hand. What is it?
It's equally comfortable in an orchestra and a geometry textbook. What is it?
I am your mother's brother's only brother in law. Who am I?