On the morning before the seder, one may eat chametz until the fourth hour of the day*. Before the sixth hour of the day, however, any chametz that remains in one’s possession (for which a sale of chametz to a non-Jew has not been arranged), including the customary ten pieces of chametz that are purposely hidden, to be found during the formal search the evening before, are burned to the point where it is inedible even to an animal. In many cities with significant Jewish communities, official sites (at times, supervised by local firefighters) are organized.
If one is in a situation in which one cannot burn the chametz (for instance, in a dorm), it is acceptable for the chametz to be destroyed in a different manner. The sages also mention crumbling it and throwing it to the wind or casting it into the sea. A more modern solution is to make it inedible by spraying it with a household chemical such as bleach, or flushing it down the toilet.
Following the destruction of the chametz, a second nullification of ownership is recited. (The first one is recited on the previous evening immediately following the search for chametz): “Any chametz or leaven that is in my possession, whether I have recognized it or not, whether I have seen it or not, whether I have removed it or not, should be annulled and become ownerless like dust of the earth.”
*This time is calculated using “Jewish” time, hence the exact times vary by location, so please check with your local synagogue or rabbi.