He evidently tried to find something to say, but failed.

Boris on the contrary at once found his footing, and related quietly and humorously how he had known that doll Mimi when she was still quite a young lady, before her nose was broken; how she had aged during the five years he had known her, and how her head had cracked right across the skull.

Having said this he glanced at Natasha.

She turned away from him and glanced at her younger brother, who was screwing up his eyes and shaking with suppressed laughter, and unable to control herself any longer, she jumped up and rushed from the room as fast as her nimble little feet would carry her.

Boris did not laugh.
"You were meaning to go out, weren't you, Mamma?
Do you want the carriage ?" he asked his mother with a smile.
"Yes, yes, go and tell them to get it ready," she answered, returning his smile.
Boris quietly left the room and went in search of Natasha.