One evening, you are sitting with your friend when his son walks in and asks,

What’s the time, dad?

Your friend looks at his watch and answers, Twenty to ten.

Exactly thirty minutes later the son enters again and you hear the same conversation between son and father as before.

Assuming your friend is telling the truth on both occasions and that his watch is keeping correct time throughout, how can this be possible?

This is one of many puzzles and games in Stuff and Nonsense – on sale at Amazon.