During the rainy season, Subir observed that a large quantity of natural and free water was being wasted. Being a mechanical engineer, Subir started to work on a device that was to be installed in underground water tanks, for collecting the rain water, freezing it and creating a surplus water resource. He prepared a prototype and started the trials on this device. He was happy that he succeeded in converting his idea into reality. Identify the idea field that provided him an insight for developing the product: