Right in the middle of the town are some long ponds . They are only about half a foot deep. The men of Symi dug out the earth to make the ponds. Over each pond is a low cover. It is made of glass that you can see through. At night, sea water is taken into the ponds. The next day, the hot sun shines through the covers. The sun light turns the water into vapor  and the vapor goes up from the ponds. The salt is left behind. As it goes, the vapor became mist. On the inside of each cover, the little drops of mist run together to make big drops. These drops run down the sides of the cover, filling other ponds with fresh water. The salt left in the ponds is washed out each night. Then more sea water is taken in. The factory is ready for the next day's work. The covers and other parts of the factory cost very little. Sea water and sun cost nothing. Most people of Symi are poor. So to them this new way of getting fresh water seems wonderful. The men of Symi dug some long ponds  _  .
in order to keep sea water in order to get hot water inside in order to fill them with fresh in order to get fresh water