In the days before watches were invented, clocks were valuable items. There was a man who had one clock in his house. It kept good time, but one day he found that it had stopped. He had no idea what the correct time was. He walked to the next valley to visit his friend who had a clock showing the right time. He spent a little while chatting with his friend then he walked home. He did not know the exact length of the journey before he started. How did he manage to set his one clock correctly on his return?