You won't need a bag of ice to keep your potato salad cold if you have a handy cooler that plugs into your cigarette lighter. It uses a unique process known as the Peltier effect, a thermoelectric effect, to produce cold temperatures. It's pretty neat, and something we haven't discussed yet.
Named after the French 19th century physicist who discovered it, you can create the Peltier effect yourself using a battery, two pieces of copper wire, and a piece of bismuth or iron wire. Attach the copper wires to the two poles of the battery, and then connect the bismuth or iron wire between the two pieces of copper wire. (The bismuth/iron and copper have to be touching -- it's this connection that causes the Peltier effect.)
The junction where current flows from copper to bismuth will start to get hot, and the junction where current flows from bismuth to the copper junction will get cold. The maximum temperature drop is about 40 degrees Fahrenheit (22.2 degrees Celsius) from the ambient temperature at the hot junction.
As you'd expect, in an electric cooler the hot junction is placed outside the unit, and the cold junction is placed inside. To amplify the effect, coolers contain lots and lots of junctions.
Electric coolers aren't the only unique inventions out there designed to chill your lunch. Solar powered refrigerators are another option. If you plan to spend time camping (or want to start your own hot dog stand), you may want chilled beverages but not have the electricity to power a standard refrigerator. By now, you won't be surprised that a number of energy solutions can provide power to refrigeration systems. In a solar powered refrigerator, a simple solar panel does the honors. Using the sun's rays to make something cold? Now that's ingenious.