Using the Style Brush
The Style Brush changes its appearence depending on the style.
The Style Brush tool allows you to copy a style from one graphic and apply that style to other graphics.

The first time you use the Style Brush it will appear plain:
Clicking on a graphic with a red fill will copy the style to the Style Brush:

Clicking on another graphic will apply the red fill to that graphic.

Applying a style will with the Style Brush will not change the shape of a graphic, or the endings for a line.

You an copy a style from a shape and apply it to a line and only style types they have in common will be applied (e.g. stroke color, stroke thickness).

To copy a new style, hold the Option key (the cursor will change back to an empty looking brush) and click on another graphic in the document.