What sparks the beginning of sex?

What sparks the beginning of sex? It seems it can start practically from anything. From a smell, a glance, a touch, a word, a thought, an emotion, a color, or an image. From a sound. From vibrations. From desires within. From a text. Or simply from a wandering sensation in the body. And sometimes sex begins with breathing - when you start noticing the movements of the chest in front of you and hearing the inhale and exhale: yours or your partner's. The inhale and exhale that suddenly cease to be invisible and soundless. And this breathing is sometimes so impossible to disguise that everything simply explodes in a moment, like a supernova flash. And that's it! Sex needs a spark. A spark that is carved by the body. A living body that is capable of feeling and wanting.