Most of the flowers in nature are red, orange and yellow. If we have seen a black flower, it's chance in a million.

    People have made a survey to colors of more than four thousand kinds of flowers and discovered that only eight of them are black.

    As we know, sunlight is formed by seven colored lights. The wavelength of each light is also different, so the quantity(量) of heat in each light is also different. Flowers, especially their petals(花瓣), are easy to be harmed by high temperature.

    Black flowers can take in the light of all the wavelengths that make them dry up because of high temperature, so black flowers can hardly resist(抵抗) sunlight. But red, orange and yellow flowers can protect themselves from sunlight by reflecting(反射) the red light, orange light and yellow light, each of which has a large quantity of heat.

    Do equations and historical( 历史的) facts give you a headache? A change in the school timetable(课程表) may be the cure.

    This  suggestion  is  based  on  a  recent  study  carried  out  by  scientists  from  Royal  Holloway, University of London, the UK. They found that teenagers could get higher marks in math if they studied in the mornings and better grades in history if they were taught in the afternoons.

    The researchers studied students' grade reports, class timetables and absence rates( 缺席率) at a middle school in Bulgaria over nine years. The results showed that when teenagers had math classes earlier in the day, they scored about 7 percent higher in tests than when taught in the afternoon. Lower-ability students feel stressed most when they were studying math in the afternoon. In history, students taught in the afternoon scored about 6 percent higher than when they were taught in the morning. However, there was no sign that similar patterns( 模 式 ) work for other subjects such as languages or geography.

    Velichka Dimitrova, who did the study, said the findings show that we perform( 表 现 ) better at different times of the day. “In the morning, our brains are better and fresher, so we are better at doing something like problem solving, where we require more speed and attention” Dimitrova told The Telegraph. “In the afternoon, it seems that this process slows down. On the other hand, history is more suitable for the afternoon, when we are more creative and open to discussion,” she said. Subjects such as foreign languages require both memory and creativity. This is perhaps why the time of day had no influence on language learning.

    Time-of-day influences such as these are believed to have something to do with the body clock.

    This is the system in our bodies that controls when we need to sleep. Some schools in the US and UK have tried later school starting times for teenagers, whose body clocks change as they grow into adults. Dimitrova believes schools should jump at this chance to improve their students' performance just by changing their timetables.

    What was discovered by accident? The answer is penicillin (青霉素). It kills germs. By killing germs, it saves lives. Suppose you are sick. You go to a doctor. She examines you. She says you have a “staph” infection (葡萄球菌感染). She gives you some medicine. You take it. The medicine knocks out the staph. Soon you are well. Before penicillin, this would not happen. Staph was almost sure death.

    Everyone wanted a medicine. Laboratories worked day and night. They grew the staph in small dishes. Then they tried to kill it. Nothing worked.

    The laboratory dishes had covers on them. They kept things from falling into dishes. Molds (霉菌) were a big worry. They are always in the air. You can't see them. They're too small. There are thousands of different molds. Molds can make an experiment (实验) fail. That's why dishes are covered.

    Dr. Alexander Fleming was working to kill the staph germ. He worked for years. One day he took a cover off a dish. He looked inside. There was a thick growth of staph germ. There was also some mold. Then he saw something strange. Where the mold was, there was no growth of staph. This is what Fleming probably thought. “By accident, I found a mold to kill the dreaded staph.”

    This is how penicillin was found. But here's the real miracle (奇迹). There are thousands of kinds of molds. But only one kind can kill staph. The mold must have fallen into the dish a few days before. The cover was probably off only a few seconds (秒). In those few seconds the right mold fell into the right dish. Another man might have thrown the dish away. But Fleming was very careful and smart. He understood what the mold did. How lucky the humans were!

    Thousands of years ago, people didn't know that the Earth moved. They thought the sun really did move across the sky. The Greek god Helios was said to drive the sun across the sky with his chariot (战车). But now, the way people think has changed. Nowadays, it is the sun that drives a "chariot" around the sky.

    What do I mean by this? Well, quite simply, there is now a plane (or "chariot") that is powered by the sun. It has special parts on its wings. These parts absorb (吸收) power from the sun. They absorb so much power that the plane can still fly even at night. In Greek mythology (神话), Helios had to take a break at night. It seems we're finally even more powerful than the gods themselves.

    This plane is called Solar Impulse. It will fly across the US in May. This is very important for the future of the world.

    Why is solar energy (太阳能) important? Well, other kinds of energy are known to cause problems. If you live in China, you've probably noticed the pollution. This is caused by burning coal and gas to make energy. The heat and chemicals created by burning these things give us power. But if we use power from the sun, we don't have to burn anything and there will not be any pollution.

    Solar energy is not entirely ready yet. It needs to be improved. But the best thing about it is that, unlike coal and gas, it doesn't run out. The Solar Impulse plane could pretty much keep flying forever. Of course, the sun's light will eventually run out too one day, but not for another 5 billion years. That's a pretty long flight.