We know we're supposed to be talking about symbols and motifs here, Shmooper, but can we take a break here to talk about a dramatic principle really quick? That baseball that Ferris's catches—you know, the foul ball at the Cubs game—is totally an example of Chekhov's gun.
No, not the dude from Star Trek. We're talking about writer Anton Chekhov, who said to get rid of anything that's not essential to the story. Like if we take the time to see a boy catch a foul ball at a Cubs game, for example, that ball better have some significance later on.
Chekhov's gun refers to an item in the story that seems unimportant when we're first introduced to it, but becomes majorly important later on. In Ferris's case, that foul ball that he catches at Wrigley Field saves his hooky-playing hide at the end of the story when he throws it at his stereo with strikingly good accuracy, turning it off just before his parents enter.
(Get it? Strikingly good? Because it's a baseball? Yeah, we're sorry.)