Do you drink water that's been left sitting out overnight or even for another day? Have you noticed it tastes different?

    Tap water that has been left to sit out slowly begins to acquire(获得) off taste. Many people think that this is because of microorganisms. But that's not what makes old water taste not fresh. For that we can thank carbon dioxide. After about 12 hours, tap water starts to go flat as carbon dioxide in the air starts to mix with the water in the glass, lowering its pH and giving it an off taste. But it's most likely safe to drink.

    However, back to those microorganisms. If you use a dirty glass day after day, there is more of a chance of bacteria making themselves known; a risk that increases if you share the glass with another mouth as well. But assuming(假设) you use a fresh glass every few days, you probably won't have a problem unless the glass has been touched by dirty fingers, and especially if those dirty fingers went unwashed after using the bathroom.

    As for plastic water bottles that have been left out in the sun or in the car, step away from the bottle, warns Dr. Kellogg Schwab, director of the Johns Hopkins University Water Institute. "A chemical called biphenyl-A, or BPA, along with other things used to make plastic can leach(过滤) into your water if the bottle heats up or sits in the sun," he explains. BPA, as you probably know, has been linked to everything from heart disease to cancer. Schwab also adds that plastic used for commercial bottled water isn't meant to be washed or refilled, so use only one time and recycle. Or don't buy them at all; use refillable water bottles instead.

    There are lots of insects that farmers hate. But there also are some they like. They protect crops against damage from other insects. A good example is the lady beetle, which is also known as the ladybug(瓢虫)．

    Lady beetles are a natural control for aphids(蚜虫). Lady beetles are red, orange or black. They often have black spots, though some have light colored spots. Different kinds of lady beetles have different numbers of spots. There are lady beetles with four, five, seven and fourteen spots.

    Many of the well-known kinds of lady beetles come from Asia or Europe. They now are common throughout the United States.

    American scientists imported one kind of lady beetle, the multicolored Asian lady beetle, as early as 1916. They released them as an attempt to control some kinds of insects. Over the years, the beetle has become established, possibly helped by some that arrived with imported plants on ships.

    Experts say over 450 kinds of lady beetles are found in North America. Some are native to the area. Others have been brought from other places. Almost all are helpful to farmers.

    The Asian lady beetles now in the United States probably came from Japan. The Asian lady beetle eats aphids that damage crops like soybeans, fruits and berries.

    In the southern United States, Asian lady beetles have reduced the need for farmers to use reskilling poisons on pecan trees. This popular tree nut suffers from aphids and other pests that the lady beetles eat.

    But some people say the Asian lady beetle has itself become a pest. They worry that the lady beetles may eat their late autumn fruit crops.

    Experts say Asian lady beetles may appear in large numbers in some years. But they say the insects are too helpful to be considered as pests.

    I often dreamed about Pisa when I was a boy. I read about the famous building called the Leaning Towers of Pisa(比萨斜塔）. But when I read the word Pisa, I was thinking of pizza. I thought this tower must be the best place to buy pizza in the world.

    Many years later, I finally saw the Leaning Tower. I knew then that it was Pisa and no pizza and that the tower got its name because it really did lean to one side. Some people want to try to fix it. They are afraid that it may fall over and they don't want it to lean over the city.

    I don't think it a good idea to try to fix it. The tower probably will not fall down. It's 600 years old. Why should anything happen to it now? And, if you ask me, I like what it looks like. It seems to say nothing is perfect. Imperfect things may be more interesting. Why is it so famous? There are many other older, more beautiful towers in Italy but Pisa tower is the most famous.

    As smog forced students to stay home, the online learning industry saw a sharp increase in consumers.

    During the air pollution red alert from Dec. 8 to 10 last winter, the Beijing Commission of Education ordered all kindergartens, primary schools and middle schools to suspend classes (停课). But the commission urged that, "Teachers should guide students to make full use of digital materials and conduct online learning at home".

    According to the China Education and Research Network, the number of new users of 17zuoye. com, an online homework platform, tripled (增至三倍) on Dec. 8 compared to the previous day.

    Online courses companies also adopted special measures to meet students' demand for Internet learning during the red alert period. For example, New Oriental offered free online English classes for three days for school students as well as online question answering services.

    Recent years have seen the popularity of online courses increase sharply in China. Massive Online Open Courses (MOOC), for example, originated in the United States but have become widespread in China since 2013. Some Chinese universities, including Tsinghua University and Peking University, have started their own MOOC platforms. "MOOCs have enlarged the time and space of teaching, fired up learners' interest, helped more people benefit from high quality educational resources and accelerated reform in many aspects of teaching," an official at the Ministry of Education told China Daily.

    With the New Year weeks approaching, you're looking forward to watching the splendid fireworks shows that will welcome 2019 worldwide. Unfortunately, visually impaired and blind people can't experience this joyful celebration. That may change soon thanks to Feeling Fireworks, a fireworks experience invented by the Disney Research Lab in Switzerland.

    To experience the show, users stand before a large flexible screen, place their hands at the base of the screen and move them around to feel the fireworks. Alternatively, their hands can be situated in the center of the screen, where the initial explosions happen, and then moved across to explore other fireworks.

    As the fireworks begin to explode, five nozzles (喷嘴) at the back of the screen start to shoot water, creating movement of water imitating the show. One specializes in the "blooming flower effect", another reproduces the "crackle" effect, and the rest take care of rockets and explosions. A computer controls the timing, and Feeling Fireworks allows users to experience fireworks similar to those in the sky.

    Paul Beardsley, who led the research team, says, "We want blind, visually impaired, and sighted people to all try Feeling Fireworks, and to have a shared and enjoyable memory of a fireworks evening." And the screen displaying the vivid images created by water makes it fun for everyone.

    The low-cost technology is still in its early stage, with only a 66 percent success rate. However, the team plans to continue improving the experience and believes the day when everyone will be able to enjoy the thrill of fireworks shows is not far. When ready, Feeling Fireworks will initially be available only at the Disney theme parks, and then hopefully, at fireworks shows worldwide.

    Madame Marie Curie famously won two Nobel Prizes, but many other women have also been awarded the prize, too. Here are their stories.

    Selma Lagerlof

    Selma Lagerlof was a Swedish author and teacher. She published her first novel, Gosta Berling's Saga, at the age of 33. She was the first female writer to win the Nobel Prize in Literature which she was awarded in 1909. Additionally, she was the first female to be granted membership in the Swedish Academy.

    Gerty Theresa Cori

    Gerty and her husband, Carl Cori, met in Prague and lived in Austria before immigrating to the United States in 1922, where the two medical doctors worked together at Roswell Park Comprehensive Cancer Center in New York. In 1947, Gerty and Carl were awarded the Nobel Prize in Physiology or Medicine, making Gerty Cori the first woman to hold the honor.

    Maria Goeppert-Mayer

    In 1942, Maria Goeppert-Mayer joined the Manhattan Project. From there, she moved on to Los Alamos National Laboratory, then to Argonne National Laboratory, where Goeppert-Mayer developed the nuclear shell model. For this, she shared the 1963 Nobel Prize in Physics with J. Hans D. Jensen and Eugene Paul Wigner.

    Dorothy Crowfoot Hodgkin

    Dorothy Hodgkin's mother encouraged her love of science as a child, and at age 18, she began studying chemistry at a women-only Oxford college. Her work on mapping vitamin B12 earned her the Nobel Prize in Chemistry in 1964.