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