Paul has to wear extremely thick glasses because his vision is so poor. No, you don't understand: these things are crazy thick. As Paul says, "My glasses are so thick…" (Prologue.1.9)
Our friendly Shmoop audience yells, "How thick are they, Paul?
They're so thick, that when people promise to stick together 'through thick and thin,' instead they sometimes say, 'through Paul's glasses and thin!'
They're so thick, that he has to take them off to weigh himself, or they add 20 pounds!
They're so thick, that if he ever looks "directly into the sun with these glasses, [his] eyeballs will burst into flame, like dry leaves under a magnifying glass" (Prologue.1.9)
Oh, we just kill ourselves.
But oddly enough, with those thickeroo glasses, he is the only one able to see things others can't—or won't. His glasses symbolize his ability to see the truth better than most people. They also remind us of the event that gave him this insight into the behavior of those around him.