Students these days often have a lot of worries. Sometimes they have problems with their schoolwork, and sometimes with their friends.      Some people think the worst thing is to do nothing. Laura Mills, a teenager from Washington, agrees. “Problems and worries are normal in life,” says Laura. “But I think talking to someone helps a lot. If we don't talk to someone, we'll certainly feel worse.”
    Laura once lost her purse, and worried for days. She was afraid to tell her parents shout it. She even walked three miles to school each day because she didn't have any money. She just kept thinking. “If I tell my parents, they'll be angry!” But in the end, she talked to her parents and they were really understanding. Her dad said he sometimes made careless mistakes himself. They got her a new purse and asked her to be more careful. “I will always remember to share my problems in the future!” Laura says.
       Robert Hunt advises students about common problems. He feels the same way as Laura. “It is best not to avoid our problems. We should always try to solve (解决) them.” He thinks that you can first find someone to talk to. This person doesn't need to be an expert (专家) like himself. Students often forget that their parents have more experience than them, and are always there to help them.
    In English, we say that sharing a problem is like cutting it in half. So you're halfway to solving a problem if you talk to someone about it!

                                                                                           火 箭

      中国科学技术馆新馆，中国古代传统技术展厅，陈列着反映中华民族灿烂文明成就的珍品，介绍四大发明及天文、陶瓷、建筑、青铜、冶铸、纺织、刺绣、中医中药、机械、手工艺等学科内容。并陈列有古代的火箭、战船，天文学的浑仪、简仪，设计精巧的水磨、水车、指南针等。

       中国人首先发明火箭这样一个事实已经被世界所公认。宋朝的时候出现了很多烟花爆竹，这些爆竹也是喷气式的原始火箭。明朝时期的毛元仪在他的著作里面就记载了30多种火箭的基本原理和结构，其中有飞火飞天、火龙出水等。中国是从明朝开始发射火箭的，大明史嘉靖年间，万户进行最早的火箭升空试验（如图1所示），成为现代载人飞行先驱，现在月球和火星均有以其名字命名的环形山．

现代火箭的飞行原理与万户的飞行试验原理基本相同。

       现代火箭大多是利用喷气发动机升空的，喷气发动机将推进剂贮箱或运载工具内的反应物（推进剂）燃烧后变成高速气流，高速气流对火箭产生巨大的反作用力使火箭加速升空。

       最常见的火箭燃烧的是固体或液体的化学推进剂。推进剂燃烧产生热气，通过喷口向火箭后部喷出高速气流。火箭自带燃料和氧化剂，而其他喷气发动机仅须携带燃料，燃料燃烧所须的氧取自空气中。所以，火箭可以在地球大气层以外使用，而其他喷气发动机不能。

       人类要在太空中走的更远，就需要不断提高火箭运行速度增大推力，科学家们为此做了大量的研究和反复的实验．提高火箭运行速度和增大推力的主要措施包括：减小火箭质量（例如三级火箭升空过程，燃料燃尽后逐级脱落，以减小火箭的质量）、增大喷出气流的速度、改善推进剂等。

       火箭远离地球后，质量和重力影响不断下降，火箭速度也因此越来越快。

       从1970年4月，长征一号火箭成功发射东方红1号卫星开始到现在，我国火箭研究与应用处于世界领先水平，火箭的稳定性和运载能力等各项性能都大大提高。

热现象在一年四季中随处可见，下列说法中正确的是（    ）