How does a potato catch fire in microwave?

A potato does not usually catch fire in the microwave. If a potato is placed in the microwave for an extended period of time without any kind of cover or wrapping, it may eventually reach its flash point and ignite, but this would generally take several minutes. Potatoes and other vegetables also sometimes give off steam depending on the variety, which can cause arcing and thus a fire if the microwave environment is not clear of any metal or aluminum foil containers or other contaminants.