I'm having a problem with an assignment for class. The thing I draw last is always on top of everything else I've drawn, regardless of position. How do I make things that are positioned behind other things show up behind them? For example, I drew 3 rectangles: one in the XY plane, one in the YZ plane, and one in the XZ plane. They should all intersect at the origin and along each axis. But when they draw, they don't intersect at all. The first one I draw is on the bottom, and the last one is on top of all the others. How do I get things in the foreground to draw on top of things in the background?