Energy saving is easily the hottest word in the 21st century. We must save energy not only for themselves, but also for our children, our children's children, etc. Nowadays the amount of PM2.5 is steadily high. That is because of the waste gas discharged by cars. The best solution is that, wherever you go, you had better go by bus instead of driving. This can lessen the pollution caused by cars. We also should send e-mails instead of writing letters to our friends. That is what we can do to save energy.