Basics
An ice box, or cooler, is a simple, portable device for keeping food and drinks cold. A block or bag of ice is kept in the cooler, along with food. The ice box is kept closed except for when something is needed from inside of it. Ideally, it is also kept out of direct sunlight. If it is used correctly, an ice box can keep food at near-freezing temperature for anywhere from a couple of hours to several days as the ice slowly melts. After that, the water needs to be drained out and the ice needs to be changed.
Keeping the Food Cold
Ice takes a certain amount of heat energy to melt. As it absorbs this energy, the ice remains at the same temperature but gradually changes to liquid water. Coolers are very well insulated, which means that heat flows into the cooler from outside very slowly. A big block or bag of ice takes a lot of energy to melt. While it is melting, the ice keeps the cold water and food that is touching it at about 32 degrees Fahrenheit, or freezing temperature. As long as new ice is added before the ice inside has completely melted, the food will stay near freezing temperature.
Insulation
Heat is transmitted in two ways convection and conduction. Convection means airflow. If the cooler is kept closed, warm air cannot flow over the ice, meaning that little heat is lost through convection except when the ice box is open. The rest of the heat is lost through conduction, or heat flowing directly through the walls of the ice box. That is why the walls are made out of insulators, or materials that do not conduct heat well. Styrofoam, one of the most common insulators, has small air pockets inside of it. Air is a very poor conductor, so these pockets stop heat outside of the cooler from flowing through the sides and top into it. Heat does slowly leak into the ice chest, but it can take days for enough heat to seep in to completely melt the ice.