When a girl is vertical, the boobs are visible, but when she is horizontal, laying with her back on the floor, the boobs disappear. Much like a soldier in camouflage attire, standing in tall grass, but when down low, the solider becomes almost invisible. This is a result of a lack of fatty matter behind the nipple to support a round shape in such a position. Normally the fatty tissue will sag out to the sides of the chest, to create a rounding of the boob shape, to give it a large circumference, however this doesn't see to happen in this case.