Sixteen months ago, a Miami teenager carrying a cell phone, a pack of Skittles, and a soft drink was shot to death in his father's Central Florida town by a self-appointed neighborhood watchman. The case soon erupted into a referral on race -- because the victim, Trayvon Martin, was black and the shooter, George Zimmerman is white and Hispanic -- and Florida's self-defense laws.

This morning, Zimmerman's trial on second-degree murder finally kicks off in Sanford with jury selection. We've run down some key questions in the case.

1. Who yells "Help!" in the 911 call?

The biggest question in the entire trial -- an issue that the defense and prosecution will each spend weeks arguing -- is whether Zimmerman or Martin was the aggressor in the struggle that ended in gunfire. Zimmerman claims the teenager attacked him and beat his head against the ground until he had to shoot in self-defense; prosecutors say Zimmerman followed the teen and then wrestled him before killing him.

The strongest piece of evidence for either side could be a 911 call made while the men were struggling. In the background, someone yells "Help!" or "Help me!" a total of 14 times. Was it Martin or Zimmerman? Experts have already weighed in for both sides, and the judge has yet to rule whether she'll allow voice experts to testify on the matter during the case.