【正文】 one knows exactl y how the earth began, as i t happened so long ago. H owever, acording to a wi del y accepted theory, the universe began wi th a “ Bi g Bang “ that threw m ater i n al di recti ons. After that, atoms began to form and bi ng to create stars and other bodi es. For several bi l i on years after the “ Bi g Bang “, the earth was sti l just a cl oud of dust. What i t was to be e was uncertai n unti l between 4. 5 and 3. 8 bil li on years ago when the dust settl ed i nto a soli d gl obe. The earthenware becam e so vi olent that i t was not cl ear whether the shape woul d l ast or not. It expl oded l oudl y wi th fi re and rock. They were i n tim e to produce carbon, ni trogen , water vapour and other gases , whi ch were to m ake the earth’s atm osphere. What i s even m ore im prtant i s that as the earth cool ed down, water began to apear on i ts surface. Water had al so appeared on other pl as li ke Mars but, unli ke the earth, i t had di sappeared later. It was not imm edi atel y obvi ous that water was to be fundam ental to the devel opm ent of l i fe. What m any sci enti sts beli eve i s that the conti nued persence of water al l owed the earth to dissolve harm ful gases and aci ds i nto the oceans and seas. Thi s produced a chain reacti on, whi ch m ade i t possi bl e for li fe to develop. Many m il i ons of years l ater, the fi rst extrem el y smal pl ants began to appear on the surface of the water. They m ul ti pli ed and fi l ed the oceans and seas wi th oxygen, whi ch encouraged the l ater devel opm ent of earl y shel l fi sh and al sorts of fi sh. N ext, green pl ants began to grow on land. They were fol owed i n tim e by l and anim al s. Som e were insects. O thers, cal l ed am phi bi ans, were abl e to li ve on l and as wel l as i n the water. Later when the pl ants grew i nto forests, repti l es appeared for the fi rst ti m e. They produced young general y by layi ng eggs. After that, som e huge anim als, cal ed di nosaurs, devel oped. They l ai d eggs to and exi sted on the earth for m ore than 140 m il li on years. H owever, 65 m il li on years ago the age of the di nosaurs ended. Why they suddenl y di sappeared sti l rem ai ns a m ystery. Thi s di sappearance m ade possi bl e thief ri se of m amm al s on the earth. These anim al s were di fferent from al l li fe form s i n the past, because they Gavi n birth to young baby anim als and prodeced m il k to feed them. Fi nnal y about 2. 6 mi l i on years ago som e sm al cel ver anim als, now wi th hands and feet, appeared and spread al over the earth. Thus they have, i n thei r turn, be e the m ost im portant ani m al s on the pl a. But they no they l ooki ng after the earth very wel. They are putti ng too m uch carbon di oxi de i nto the atm osphere, whi ch prevenrs heat from escapi ng from the earth into space. As a result of thi s, m any sci enti sts beli eve the earth m ay b e too hot to l i ve on. So whether li fe wi l conti nue on the earth for mi l i ons of years to e wi l depend on whether thi s probl em can be solved. A vi si t to the m oon Last m onth I was l ucky engough to have a chance to make a tri p i nto space wi th m y fri end Li Yanpi ng, an astronom er. We vi si ted the m on i n our spaceshi p! Before we l eave, Li Yanpi ng expl ai ned to m e that the forece of the gravi ty woul d change three tim es on our journey and that the fi rst change woul d be the m ost powerful. Then we were off. As the rocket rose i nto the air, we were pushed back i nto our seats because we were tryi ng to escape the pul l of the earth’s gravi ty. It was so hard that we coul d not say anythi ng to each other. Gradual ly the weight l essened and I was abl e to tal k to him. “ Why is the spaceship not fal li ng back to the earth? O n the earth if I fal l from a tree I wil l fal l to ground. ” I asked. “ We are too far from the earth now to fel i ts pul l, “ he expl ai ned, “ so we feel as i f there i s no gravi ty at al . When we get cl oser to the m on, we shal feel i ts gravi ty pul li ng us, but it wi l not be as strong a pul l as the earth’s. “ I cheered up im m edi atel y and fl oated wei ghtl essl y around i n our spaceshi p cabi n watchi ng the earth be e sm al er and the m oon larger. When we got there, I wanted to expl ore im medi atel y. “ Com e on,” I sai d. “ If you are ri ght, m y m ass wi l be l ess than on the earth because the moon i s smal er and I wil l be abl e to move m ore freel y. I m i ght even grow tal l er i f I stay here l ong enough. I shal l certai nl y wei gh l ess!” I l aughed and cl im bed down the steps from the spaceshi p. But when I tri ed to step forward, I found I was cari ed twi ce as far as on the earth and fel over. “ O h dear, ” I cri ed, “ wal ki ng does need a bit of practi ce now that gravi ty has changed. ” After a whi l e I got the hang of i t and we began to enjoy ourselves. Leavi ng the m on’s gravity was not as painful as l eavi ng the earth’s. But returni ng to the earth was very fri ghteni ng. We watched, am azed as fi re broke out on the outsi de of the spaceship as the earth’s gravi ty increased. Agai n we were pushed back hard i nto our seats as we cam e back to l and. “ That was very exhausti ng but very exciti ng too, ” I sai d. “N ow I know m uch m ore about gravi ty! Do you thi nk we coul d bisi t som e stars next ti m e?” “O f course,” he sm il ed, “ whi ch star woul d you l i ke to go to?” Caught by a bl ack hol e The spaceship warned Li Yanpi ng and m e to be ready as we were approachi ng the “black hol e”. We coul d see som e fai nt l i ghts around what l ooked li ke an em pty space. Li Yanpi ng sai d, “ That em pty hole i s l i ke a m outh al ways needi ng to be fed. Those l i ghts are thi ns that are being pul ed i nto tim e by the gravity of the hol e.” We watched with i nterest and am azem ent at the li ghts whi ch seem ed to be goi ng fai nter and fai nter round the edge of t