The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole（北极）is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere（半球）. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    Do you know of anyone who uses the truth to deceive(欺骗)? When someone tells you something that is true, but leaves out important information that should be included, he can give you a false picture．

    For example, someone might say, “I just won a hundred dollars on the lottery(彩票). It was great. I took that dollar ticket back to the store and turned it in for one hundred dollars!”

    This guy is a winner, right? Maybe, maybe not. We then discover that he bought $ 200 worth of tickets, and only one was a winner. He's really a big loser!

    He didn't say anything that was false, but he left out important information on purpose. That's called a half-truth. Half truths are not technically lies, but they are just as dishonest.

    Some officers often use this trick. Let's say that during Governor Smith's last term, her state lost one million jobs and gained three million jobs. Then she tries another term. One of the persons who don't support her says, “During Governor Smith's term, the state lost one million jobs!” That's true. However, an honest statement would have been, “During Governor Smith's term, the state had a net gain of two million jobs.''

    Advertisers will sometimes use half—truths. It's against the law to make false statements so they try to mislead you with the truth. An advertisement might say, “Nine out of ten doctors advised their patients to take Yucky Pills to cure toothache. ”It fails to mention that they only asked ten doctors and nine of them work for the Yucky Company.

    This kind of thing happens too often. It's a sad fact of life: Lies are lies, and sometimes the truth can lie as well.

    There is a popular belief that goldfish only have a three-second memory. But a 15-year-old schoolboy from Adelaide has just finished an experiment to tell us that it is not true. He shows everybody that the goldfish is smarter than we think.

    "I don't believe that they had a three-second memory because animals need their memory, so they build up over time a knowledge of where the food is," said Rory Stokes, a student at the Australian Science and Mathematics School.

    He did the experiment in a small tank(鱼缸)of goldfish. "I decided to get a bit of red Logo and just feed them next to that. Every day I'd put it in and spread food around it." He said.

    "At first they were a bit scared of it, but by the end of the three weeks, they were actually almost coming before I put the food in."

    After leaving the fish alone for a week, Rory placed the red Logo block in the tank again.

    "They remembered perfectly well," he said.

    "They actually had a time faster than the average(平均)of the three feeds before I left."

    The goldfish showed that they not only could store information, but also had the ability to get it back at a later date.

    Culum Brown, a researcher at Sydney′s Macquarie University, has studied fish behavior for more than ten years.

    He says his studies of Australian native fish show fish are smart animals that know how to avoid enemies and catch food like any other animal.

    "The thing that I really liked about Rory′s experiment is that he not only got that classical conditioning going but the fact that he could get thorn next just to that specific coloured marker. I thought it was really good." He said.

    Today, many endangered animals live in zoos. There, scientists work to keep each species(物种) alive. How do they do that? One way is by taking cells(细胞) from animals. Then the cells are frozen(冷冻). They can be used in the future to help animals reproduce.

    The San Diego Zoo has been freezing cells for 40 years. It's also called the Frozen Zoo. Now, the zoo is the biggest of its kind, with a large gene bank. It has cells from more than 1,000 species. One of the species is the northern white rhinoceros(犀牛).

    There are only five northern white rhinoceros alive in the world now. They are unable to have their children. Some scientists there want to use the bank's frozen cells to produce another northern white rhinoceros. They want to do this before the species dies out. That could happen by 2025.

    Can the zoo save the northern white rhinoceros? Other species have been saved successfully. So it's possible for the white rhinoceros too.

    Saving the white rhinoceros would be expensive. So some people don't think using frozen cells is the best thing to do. Some wonder whether it's worth spending millions of dollars on the species that nearly dies out. Some say scientists should work on the reasons why animals become endangered in the first place.

    All week long, it seems you sit inside at school when it is fine outside. But when the weekend comes, it is very cloudy. There is rain according to(根据) the weather report.

    Scientists studied more than 40 years of weather data(数据) from around the world. It shows weekend weather is different from weekday weather in some places.

    Part of the study involved(包括) 660 weather stations in the US. At more than 230 of these places, the weather on Saturday, Sunday and Monday was different from the weather on Wednesday, Thursday and Friday. The difference was small, but the pattern was striking enough to make the scientists take notice.

    "This kind of weekly rise and fall doesn't line up with any natural cycles, "the scientists say. Instead, they think that air pollution from human activities makes the weather become different.