A school installed water refill stations and hoped to reduce the use of disposable bottles. During the first month, the number of plastic bottles in classroom bins fell. The student committee was encouraged, but it recorded three additional facts before claiming success. The weather had become cooler, so students were drinking less water overall. A nearby shop had also stopped selling one popular bottled drink. Finally, some students were bringing reusable bottles but filling them at home. The committee created a three-column record: the observed change, the evidence that might explain it, and a new check that could separate the explanations. It compared warm and cool days, counted refills at the stations, and asked the shop when sales changed. The final report concluded that the stations probably helped, but the fall in discarded bottles could not be assigned to one cause alone.