The primary reason biting on aluminum foil is painful is because of its sharp edges. When a person bites down on aluminum foil, the edges of the foil can cut the soft tissues of the mouth, including the gums, tongue, and insides of the cheeks. The pain is caused by the sharpness of the edges, which can act like tiny blades, causing cuts and abrasions.

Additionally, the act of biting on the aluminum foil can cause further damage to the mouth. The sharp edges can tear the delicate tissues, leading to bleeding and discomfort. In some cases, biting on aluminum foil can even cause small pieces of the foil to become stuck in between the teeth, which can be painful and difficult to remove.

Therefore, biting on aluminum foil is generally not recommended as it can cause cuts, abrasions, bleeding, and even potential dental problems.