Sophie was reading a story in which Frog and Toad went to the snow. Frog (or Toad, I forget which) was afraid to go sledding but took comfort and courage in the presence of his friend and both went together. Whizzing down the hill, and delighting in his experience Frog exclaimed again and again to his friend about how wonderful it was and how he could not have done it without his friend. Now the reader knows that Toad fell off the sled ages ago and that Frog is actually all alone. When this is revealed to Frog he suddenly can no longer control the sled and careers off here, there and everywhere.
Says Grandpa to Sophie: Now that's dramatic irony.
Says Sophie to Grandpa: No, Grandpa. That's a sled.