Plastic drinking water bottles are made from polyethylene terephthalate (PET), which has good strength and durability, but PET cannot withstand the scorching summer sun. Under direct sunlight, the polymer slowly degrades to form two monomers, one of which is very toxic to humans and causes a variety of symptoms; moreover, it is highly soluble in water, so drinking water from a bottle that is constantly exposed to the sun is not recommended. Water from such a bottle may taste slightly sweet. What toxic substance is formed when PET breaks down?