The pot-in-pot refrigerator, also known as a Zeer الزير in Arabic, is a refrigeration

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 device which keeps food cool without electricity

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 by using evaporative cooling. A porous outer earthenware

Earthenware

Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...

 pot, lined with wet sand, contains an inner pot (which can be glaze

Ceramic glaze

Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...

d to prevent penetration by the liquid) within which the food is placed. The evaporation of the outer liquid draws heat from the inner pot.

Although the principle behind the device has been known for centuries, one modern device was developed by lecturer Mohammed Bah Abba for use in modern Nigeria

Nigeria

Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

 and distributed by his company, Mobah Rural Horizons.
There is some evidence that evaporative cooling was used as early as the Old Kingdom

Old Kingdom

Old Kingdom is the name given to the period in the 3rd millennium BC when Egypt attained its first continuous peak of civilization in complexity and achievement – the first of three so-called "Kingdom" periods, which mark the high points of civilization in the lower Nile Valley .The term itself was...

 of Egypt, around 2500 B.C.