Toushirou just kind of rolls his eyes a bit when he sees the older boy slip; he hasn't seen the slime himself, so it just looks like somebody being clumsy to him. Whatever. It's dumb, but it happens.

He continues on his way -- one thing that hasn't changed in the past year is Granny's need for him to go out and do a few things for her -- and would have been content pass right by the the boy, except he catches what he does next. Just. Eating something you picked up off the street. Who does that?

"Most people probably know better than to eat something they just picked up off the ground."