It's impolite to make rude comments.
Tom is at an art exhibition.
Tom wants to discuss the art.
Tom turns to the person next to him and asks what they think the painting means.
Tom turns to the person next to him and says that the art is ugly.
The person next to Tom says that he is the artist and describes the meaning of his art.
The person next to him turns out to be the artist and is very offended.