The Clock
George did not have a wristwatch, but he did have an accurate clock.
However, he sometimes forgot to wind his clock.
Once when this happened, George went to his sister’s house, spent the evening with her, went back home and set his clock.
How could he do this without knowing beforehand the length of the trip?
When George left his house, he started the clock and wrote down the time it then showed.
When he got to his sisters house he noted the time when he arrived and the time when he left.
He thus knew how long he was at his sister’s house.
When he got back home, he looked at the clock, so he knew how long he had been away from home.
Subtracting from this the time he had spent at his sister’s house, he knew how long the walk back and forth had been.
By adding half of this to the time he left his sister’s house, he then knew what time it really was now.