The jurors examine and question the old man's testimony in scene 6. During the trial, the old man, who lives downstairs from the scene of the crime, testified that he heard arguing coming from the apartment above him, including a threat of murder. He then testified that he heard the sound of someone falling down. Alarmed, he ran to the window down the hall and saw the boy running out of the building. He then called the police.
This testimony makes the old man the prosecution's star witness. It certainly seems like enough information to convict the boy of the murder. However, Juror 8 wonders if the old man's story is accurate. Apparently, the witness was in a feeble state, having suffered a stroke some time earlier. He entered the courtroom walking slowly with two canes. Juror 8 does not think that the old man could have had enough time to make it to the window to see what he claims he saw. To illustrate this, he makes a diagram of the apartment on the floor. Moving at the speed of the old man, Juror 8 takes over 40 seconds to cover a distance that would have needed to have been covered in 15 seconds. Furthermore, the timing of the passing train means that it would have been near impossible for the witness to have heard what he claims he heard.
This presents a major flaw in the witness's testimony. Juror 9 suggests that the old man, feeling discarded by society, might have been inventing facts for attention. He tells the others that
A man like this needs to be recognized. To be questioned, and listened to, and quoted just once. This is very important.
All this introduces reasonable doubt into the old man's testimony. Indeed, it is this weakness in the testimony that finally convinces several jurors to acquit.