【正文】 t cl oser to the moon, we shal fel its gravi ty pul ing us, but i t wil not be as strong a pul as the earth?s. “ I cheered up imm edi ately and fl oated wei ghtlessl y around i n our spaceshi p cabin watching the earth be e smal er and the m on l arger. When we got there, I wanted to expl ore imm ediatel y. “ Com e on,” I sai d. “ If you are ri ght, m y m ass wil be l ess than on the earth because the moon is sm al l er and I wil be abl e to m ove m ore freel y. I mi ght even grow tal ler if I stay here long enough. I shal certai nl y wei gh less!” I l aughed and clim bed down the steps from the spaceshi p. But when I tri ed to step forward, I found I was caried twi ce as far as on the earth and fel over. “ O h dear, ” I cri ed, “ wal ki ng does ned a bi t of practi ce now that gravi ty has changed.” After a whil e I got the hang of it and we began to enjoy ourselves. Leaving the m on?s gravi ty was not as painful as leavi ng the earth?s. But returning to the earth was very fri ghtening. We watched, amazed as fi re broke out on the outsi de of the spaceshi p as the earth?s gravity i ncreased. Agai n we were pushed back hard i nto our seats as we cam e back to l and. “ That was very exhausting but very exciting too,” I said. “Now I know m uch m ore about gravi ty! Do you think we coul d bi si t som e stars next tim e?” “O f course, ” he smi led, “ whi ch star woul d you l i ke to go to?” Caught by a bl ack hole The spaceshi p warned Li Yanping and m e to be ready as we were approachi ng the “bl ack hol e”. We coul d see som e fai nt l ights around what l ooked li ke an em pty space. Li Yanpi ng sai d, “ That em pty hol e is l ike a m outh al ways needi ng to be fed. Those l ights are thi ns that are bei ng pul ed i nto tim e by the gravi ty 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 the black hol e. Just then the li ghts on our spaceshi p went our the and the puter stopped worki ng. What was happening? I tri ed to turn the spaceship away from the hol e but i t woul d not m ove. Woul d i t eat us too? Sudenl y the spaceshi p jum ped and began to m ove round the edge of trhe hol e too. The gravi ty of the “bl ack hol e” was pul li ng us i nto its “m outh”. As the spaceshi p m oved round the black hol e, I fel t rather si ck. My m outh went dry and I cl osed my eyes. Al the stori es about what happened when you were caught by the gravi ty of a “black hol e” seem to be true. But then the spaceshi p jum ped agai n. This tim e i t m oved si deways and away from this edge of the “bl ack hol e”. Som eone watchi ng m e woul d have seen the spaceshi p flying li ke a bi rd away from the “bl ack hol e”. But we felt nothi ng. At l ast we were abl e to l ook around us. Luckil y the puter had started worki ng agai n. We reali zed that we had discovered somethi ng new about “black hol e”. If you go i nsi de the edge of a “bl ack hole”, i t wi l “eat” you and there i s no escape。 but i f you do not, you m ay have an opportunity to escape. H ow exciting! Uni t 5 A tri p on “ The True North” Li Daiyu and her cousin Li u Qi an were on a tri p to Canada to bisit thei r cousi ns i n Montreal on the Atlanti c coast. Rather than take the aeroplane al the way, they deci ded to fly to Vancouver and then take the trai n west to east across Canada. The though that they coul d cros the whol e conti nent was exci ti ng. Thei r fri end, Danny Li n, was wai ti ng at the ai rport. He was goi ng to take them and thei r bagage to catch “The True N orth”, the cross Canada trai n. O n the way to the stati ion, he chatted about thei r tri p. “You ?re goi ng to see som e great scenery. Goi ng eastward, you?l l pas mountai ns and thousands of lakes and forests, as wel l as wi de ri vers and l arge ci ti es. Som e peopl e have the idea that you can cross Canda i n l ess than fi ce days, but they fet the fact that Canada is 500 kil ometers from coast to coast. Here in Vancouver, you?re i n Canada?s warm est part. Peopl e say it i s Canada?s m ost beauti ful ci ty, surrounded by mountai ns and the Paci fic Ocean. Ski ing in the Rocky Mountai ns and sail i ng i n the harbour m ake Vancouver one of Canada?s m ost popul ar ci ti es to li ve i n. Its popul ati on i s increasi ng rapi dl y. The coast north of Vancouver has som e of the oldest and m ost beautiful forests i n the worl d. It i s so wet there that the tres are extrem el y tal l, some measuring over 90 m etres. ” That afternoon aboard the trai n, the cousi ns settl ed down in thei r seats. Earli er that day, when they crossed the Rocky Mountai ns, they managed to catch sight of som e m ountai n goats and even a gri zzl y bear and an eagl e. Thei r next stop was Cal gary, which i s fam ous for the Cal gary Stampede. Cowboys from al over the worl d e to pete in the Stampede. Many of them have a gi ft for ri ding wil d horses and can wi n thousands of dol ars in prizes. After two days? travel , the girls began to reali ze that Canada i s qui te em pty. At school, they had l earned that most Canadi ans li ve wi thi n a few hundred kil ometres of the USA border, and Canada?s popul ati on i s onl y sl ightl y over thi rty m il i on, but now they were amazed to see such an empty country. They went through a wheatgr owi ng provi nce and saw farm s that covered thousands of acres. Afrer di nner, they were back i n an urban area, the busy port ci ty of Thunder Bay at the top of the Great Lakes. Because of the Great Lakes, they l earned, Canada has more fresh water than any other country i n the worl d. In fact, i t has onethi rd of the worl d?s total fresh water, and m uch of i t i s in the Great Lakes. That ni ght as they sl ept, the trai n rushed across the top of Lake Superi or, through the great forests and southward towads Toronto. “The True North” from Toronto to Montreal The next m orning the bushes and m apl e