Is she weighing the diapers to determine how much water they hold? If she has access to a scale, one way to do the experiment so that you don't need to know beforehand how much water the diaper holds is to weigh the dry diaper, then soak it in water until it's saturated, then weigh the wet diaper. I am not sure how old your daughter is, but the density of water is about 1g/cc. So, the weight in grams gives you how many cc's of water is in the diaper. (This is how hospitals calculate urine output in infants, by the way). Depending on her grade level, and the degree of sophistication of the course, she might need to correct the density for temperature. If it is a middle school level course, most likely a good food scale would be sufficiently accurate. (I have a digital one that is very nice). Otherwise, the lab at school should have a balance she could use.