A hot water heater
Mount a bathtub on a metal stand about a foot high. Fill the tub with water. Submerge a coil of copper tubing in the tub with one end of the tubing connected to a water spigot and the other end to a shower head inside a trailer. Build a fire under the tub. The heated water in the tub will warm the water in the tubing. Open the spigot and, voilá! Hot water flowing to your shower.
This particular masterpiece of engineering was built by my friend Paul Limon in the back yard of his weekend getaway trailer in Chambless, California. When Paul was proudly showing me his contraption, I just had to smile. I don’t know what the circular lid is for; perhaps just decoration!