"On his tenth, the last I saw him." Jack rubbed a thumb over Susan's cheek once, watching her for a long moment. "And arguing with a six-year-old over the merits of vinegar versus ketchup with chips."
A change of subject to keep the mood from worsening, much the same as he remembered Vain picking an arguement with the Doctor to keep him from brooding about the Time War. Or Rose dragging them out shopping, or insisting they visit her mum, to acomplish the same thing.