Aluminum foil is a thin metal sheet, and its behavior in the microwave is different from most other materials because of its electrical conductivity. While aluminum is indeed reflective to microwaves, it can still heat up when placed inside a microwave oven due to a phenomenon known as "electric current induction."
When microwaves interact with conductive materials like metal, the electric fields in the microwaves induce electric currents in the metal. These induced currents cause the metal to rapidly heat up, and this effect is more pronounced in thin, pointed, or sharp-edged metal objects like aluminum foil.
As the electric currents flow through the aluminum foil, they encounter areas of higher resistance, such as edges or wrinkles, causing those areas to heat up more quickly. This concentrated heating can create sparks, and in some cases, it can lead to fires or damage the microwave oven.
To avoid accidents and damage, it is generally recommended not to use aluminum foil in the microwave unless the manufacturer explicitly states it's safe for certain microwaveable containers or packaging. Instead, use microwave-safe containers or coverings to heat or cook food in the microwave. These are designed not to conduct electricity and will not cause the same heating effects as aluminum foil.