Passage Summary: Researchers compared three groups of brushers: one used the same brush all year, one sterilized their brush monthly, and one got a brand-new brush monthly. Surprisingly, the sterilization group had just as much plaque as the group that did nothing to their brushes, while the new-brush group had less.

Reasoning: N/A

Analysis: The paradox here lies in the failure of sterilization to provide any benefit over a dirty toothbrush, while a brand-new toothbrush succeeded. We are looking for a difference between a 'sterilized old brush' and a 'new brush' that doesn't involve bacteria. If the issue isn't germs, it must be the physical condition of the tool itself. An answer that suggests bristles lose their mechanical effectiveness over time, regardless of how clean they are, would resolve the discrepancy perfectly.