Suppose you find a bright yellow bike on a street corner in the city. You hop on(跨上)it and 1. But wait—isn't this stealing? No, it isn't. 2 shouts,“Stop! Thief!” That is 3 this free ride is just fine with the city.

    You 4 find hundreds of free yellow bikes in some US cities. The idea began in Portland, Oregon, in 1994. People saw a need for 5 transportation, and they wanted to help 6 pollution. So, to get citizens(市民) 7 their cars and onto pollution-free bikes, they 8 the Yellow Bike Project.

    The 9 bikes are painted bright yellow and placed throughout the city. People can hop on the yellow bike and ride to run errands(短程差事), to work 10 to school. Then, they leave the bike for the 11 rider. There have been times when bikes have been stolen, but most people 12 the rules, because they think it is not wise to 13 the bike that's already free.

    Portland's 14 was quickly loved by a lot of people. Within two years of its starts, similar programs were 15 in cities in six other states.