One man shows another the portrait of a gentleman and tells him: 'I have neither brothers or sisters, but this man's father is the son of my father.' Who is the man in the painting?

'This man's father is the son of my father...' That establishes the gentleman in the potrait as the son of the man holding the picture.

Dave is right. Lots of people get confused by this puzzle. The way to figure it out is to think about the phrase 'the son of my father'. Who would this be. It would be me. It could be my brother, but I have none, so it must be me. Now replace the phrase 'the son of my father' with the word 'me' and try it again: 'I have neither brothers nor sisters, but this man's father is me.' You should be able to work out that if the man's father is me, the man must be my son.