Answer: Leavened foods
More information: Leavened foods, also known as chametz, are strictly forbidden during Passover. Chametz is any food made with leaven/yeast/baking powder. Good examples of chametz include the likes of cake, pizza, ordinary bread (containing yeast) and cereal. The reason leaven foods are forbidden is because when the Jews were escaping from Egypt, they were in such a haste that they had very little time to allow the dough of their bread to rise.