Fill in the blank. Whitening materials penetrate the teeth enamel into the ________. Type the correct answer into the space provided.

The correct answer is "dentin."

Whitening materials work primarily by penetrating the outer layer of the tooth, known as the enamel, and reaching the underlying layer known as dentin. Dentin is less dense than enamel and contains microscopic tubules that lead to the tooth's nerve, which means that whitening agents can more effectively break down stains and discoloration present in this layer. This process allows the teeth to appear noticeably whiter after treatment, as the removal of deeper stains translates to a brighter overall appearance. Understanding this mechanism is essential for evaluating different whitening products and their effectiveness, particularly in terms of how deep they can reach within the tooth structure.