When Napoleon marched huge forces across Europe, he needed an efficient way to store provisions. A Frenchman preserved sterilized food in empty champagne bottles, an idea modified by the British, who tried tin cans. Still, canned foods sometimes spoiled, which led to experiments with refrigeration. Later, it was discovered that gases may be stored at very low temperatures in a thermos flask, a device handy for picnics, for polar explorers and for, storing large quantities of liquid oxygen and hydrogen. When lit by a spark these gases can send rockets into space.