The consumption of chametz is forbidden on the 14th of Nissan from the fourth hour of the day, but one can still derive benefit from it throughout the fifth hour too. (These refer to halachic hours, calculated from sunrise.) After the last consumption of chametz, one should clean out their teeth with a wooden toothpick, picking out anything stuck in the gaps and making sure that nothing is left. One should then wash their hands and rinse their mouth well, rubbing their teeth with their finger, because even the most minute amount of chametz is forbidden on Pesach. It once happened that there was a certain pious man, who was extremely careful with guarding the flour for his matzah, and baked it meticulously and observed all the laws of Pesach. On the eve of Pesach, as he fell asleep, he dreamed that a man was telling him: "Woe to he who ate some chametz!" He woke up and was very distraught and wept profusely, and while weeping fell back asleep. He dreamed again, and a man was speaking to him, saying: "You ate chametz! It was a small crumb that stayed between your teeth from the morning before Pesach, and in eating the matzah at the Seder, the crumb was dislodged, and you ate chametz together with your matzah!" Therefore, it is extremely important to be diligent to clean one's teeth well.