A sentence starts out like...

A sentence starts out like an onstage performance. Before the curtain even lifts, before the spotlight hits the stage, there has been an incredible amount of planning behind it.  After deciding on the piece to be performed, the producers or makers need to start the casting process. In the case of a sentence, once a topic has been chosen, we need to pick which ideas we will select to support it.  But it is only after months and months of rehearsals, or many rewrites later, that the play (or sentence) is ready to finally be presented to an audience.