People tried to keep cool or keep their food cool for ages.The Chinese invented ice cream 4000 years ago,and they have kept it in snow in underground chambers.In Baghdad in the eighth century,a king imported snow and put it in the walls of his summer home to cool the home.In 1834,Jacob Perkins built a machine to make ice in London.The first home refrigerators appeared in the early 1920. Since then,people have preserved their food in refrigerators. About the same time,air conditioning of large spaces have become possible.Since that time,people have cooled themselves with air conditioners in stores,offices,and homes.