You need to transfer heat from one fluid at 100°C to another at 0°C without mixing them. Given some tubing, you could wrap one length around another. Then, with enough wound tubing, pumping both in at one end, at the nether end you would find that both fluids came out at about 50°C. Good.
Can you do better? Can Nature? This is the nut of one (and thereafter many) of the problems Knut Schmidt-Nielsen (of Norwegian lineage, if you wondered; with a statue of him gazing at a camel on the Duke campus, if you wanted to wonder) was driven to by a sojourn in Arizona Sonoran desert half a century ago. There his naturalist's eyes found surprisingly abundant wildlife given the rigor of the environment. Consider the lowly kangaroo rat. It never drinks water and has no appetite for moisture-rich greens. But like all of us, the rat must...
