My interpretation:
I think this song is about two people who try to follow a formula for love because this is what they think love is. Then, they realize that love is so much bigger then what they originally thought. The fire, or love, consumes them. So they take out their hearts, which are nothing but "little stones" and try to make them produce what they are feeling. They strike and strike until finally the hearts produce sparks, or love. By doing this, they are breaking free from the formula and proving that there is no true way to make love happen or work.
My interpretation: I think this song is about two people who try to follow a formula for love because this is what they think love is. Then, they realize that love is so much bigger then what they originally thought. The fire, or love, consumes them. So they take out their hearts, which are nothing but "little stones" and try to make them produce what they are feeling. They strike and strike until finally the hearts produce sparks, or love. By doing this, they are breaking free from the formula and proving that there is no true way to make love happen or work.