One of the most important pieces of evidence in the murder trial in 12 Angry Men is the murder weapon. It was a knife, of course, but in the words of Juror #4, "This wasn't an ordinary knife. It had a very unusual carved handle and blade. The storekeeper said it was the only one of its kind he had ever had in stock."
In other words, it seems like a moot point that the murder weapon was the same knife that the defendant bought from a shop earlier on the night of the murder. This piece of evidence alone is pretty much enough to bring down a Guilty verdict, as Juror #4 adds, "They identified the death weapon in court as that very same knife."
But hold on just a second, because we're about to see one of the most exciting moments in the whole movie.
Just when it looks as though the knife in question will get the defendant convicted, Juror #8 pulls the exact same type of knife out of his pocket and jams it into the table in the middle of the room. And just so there's no confusion about where he got it, he adds, "I bought that at a pawn shop two blocks from the boy's house. It cost six dollars." So with this little flourish, he has proven that the knife isn't unique enough to bring a conviction on its own.
You might be thinking (along with the other jurors) that proving the knife isn't unique doesn't mean the kid is innocent. In fact, it proves very little. But Juror #8 also gets the help of Juror #5 to say that if the defendant was as skilled with knives as the prosecution says he is, he never would have stabbed downward into his father's chest. In the words of Juror #5, "Here's how. Underhand. Anyone who's ever used one wouldn't handle it any other way." Again, this testimony isn't enough to sway people. But it continues to put doubt into pieces of evidence that used to be seen as sure things.
The mysteriousness of the knife shows that what seem to be the most self-evident truths may not actually be true. It all depends on how you look at things, and which questions you decide to ask.