One of the most useful day-to-day inventions of the 20th century, the refrigerator allows our food to be stored over long periods, reducing the growth of bacteria dramatically. It was invented originally in 1922 when two students at the Royal Institute of Technology in Stockholm, Sweden, created a gas-absorption chilling cabinet. Unlike modern fridges though this device did not use an electrically driven compressor to maintain internal temperature, but instead an ingenious system of state-changing gases. After realising its potential the inventors put the refrigerator on sale. Unfortunately, it never really caught on, leaving the later electric fridge to make the jump to mass-market success.