Across the supper table

An original fiction passage set in the nineteenth century.

Mrs Dacre asked whether the journey had been pleasant. Her son said it had been uneventful. He kept his eyes on the tablecloth, tracing a crease with the tip of his knife. She waited for him to say more. Between them, the unused questions crowded the table. At last she offered him another potato, and he thanked her with a relief that made her look away.

Identify the phrase which suggests that mother and son avoid discussing important matters.