"But what on earth happened to the sidewalk." As he paused before his door on the way back, he looked carefully down at the sidewalk.

Right before the door several flags in the walk appeared to be thinly coated with some colorless specimen of slime.
"It looks as though it might be soft soap," pondered Prescott, examining the stuff more closely.

"It'll be dry in a half an hour more, but I think I had better fix it." In the basement was a barrel of sand that was used for sanding the icy sidewalk in winter.

As soon as Dick had run upstairs with the bread he went below, got a few handfuls of sand and fixed the sidewalk.
At recess Dick noticed just enough about his wrist to make him speak about it to Submaster Luce.
"Let me see it," demanded coach.

"Hm!" he muttered.