Isn't he a duck? inquired Tuppence ecstatically, as she skipped down the steps.
Tuppence missed a step as she was coming down and fell face first on the floor. 
mnli hypothesis: Tuppence missed a step as she was coming down and fell face first on the floor.  premise: Isn't he a duck? inquired Tuppence ecstatically, as she skipped down the steps.