An American group developed the rooftop ( 屋顶 ) gardening in the early 1980s.The group is known as ECHO. This idea was to help people living in cities who have no land to grow vegetables. And now it is becoming increasingly popular in many other countries. To build a rooftop garden, four things are necessary. You need a roof strong enough and four pieces of wood to build a box. Grass cutting in which to plant your garden and a thin piece of plastic on which to put the cut grass are also necessary. Begin by testing your roof to make sure it is strong enough. Cut and collect some grass. Cover the roof with a thin piece of plastic. Then you build a box from the four piece of wood. The box has no top or bottom. It should be about 8cm deep. It can be as long and as wide as needed. Put the box on the plastic. Then fill in with the cut grass. Water the grass and push it down by walking on it. After about three weeks, the rooftop box is ready for planting. Put the seeds directly into the wet grass. It is important that the grass is kept wet until the plants begin to grow. When the plants are growing, you will need to water them every day unless it rains. And birds must be prevented from eating the seeds or new plants. They will also need some liquid fertilizer ( 液体化肥 ). You can make your own liquid fertilizer if you can get waste from chickens. Put the chicken waste in a cloth bag. Then put the bag in a big container ( 容器 ) of water. After about one week, the water becomes a good liquid fertilizer. The rooftop garden is a good place for peas, tomatoes and onions. Potatoes and carrots will also grow if the box is deep enough.