The old man was a witness in a burglary trial.
The defense lawyer asked Sam, “Did you see my client commit this burglary?”
“Yes,” said Sam, “I plainly saw him take the goods.”
The lawyer asked again, “Sam, this happened at night. Are you sure you saw my client commit this crime?”
“Yes,” said Sam, “I saw him do it.”
Then the lawyer asked, “Sam, listen: you are 80 years old and your eyesight probably is bad. Just how far can you see at night?”
Sam replied, “I can see the moon — how far is that?”