One slice of bread has a green patch. Geeta cuts off that corner and throws it away, saying the…

Why

Geeta's rule works for a lot of spoiled food you can see, a bruised bit of apple, a bad patch on a vegetable, so trimming feels safe here too. But mould is not just the fuzzy patch; that patch is only the part making spores. Underneath and around it, a web of fine threads has already grown through the soft bread well past the colour, too thin to see, and some moulds leave chemicals in that region that heat will not destroy. Bread is soft and moist enough that the threads spread fast and far. A firm dry food like a hard block of cheese is different, because there the mould cannot push in and cutting a wide margin is genuinely fine. Test: hold the slice up to the light and you can sometimes see it faintly discoloured well beyond the green. Vivek is right: on soft food, a visible spot means the whole piece is likely reached.