A biologist from the National Park Service discovered a rare and unusual mammoth(猛犸) skull(颅骨) buried in a 13,000-year-old rock layer on the Santa Rosa Island, the second largest landmass in the Channel Islands, California. The fossil of the extinct giant animal is leaving many paleontologists(古生物学家) scratching their heads. Despite the fact that it's possibly the best preserved mammoth skull ever found and of high scientific importance, the species of the individual it belonged to can't seem to be identified yet—it's too big to be a pygmy(特别矮小的) mammoth and too small to have come from a Columbian mammoth. Some say it's a new species while others believe the truth lies somewhere in between.

    The first mammoths showed up in North America around two million years ago, but it was only during the last two ice ages that the Columbian species, which could grow to be up to 14 feet tall, made its way to the Channel Islands 100 miles west of Los Angeles. Once the ice melted, many populations became trapped on the island and evolved into pygmy mammoths, an endemic species(地方种) to the Channel Islands which grew only to six feet tall.

    Judging from its size, the newly found mammoth skull doesn't seem to fit any of the two species, Columbian or pygmy. To make things even more confusing, one of its two tusks(象牙) is nearly five feet long and coiled in a manner that resembles those of fully grown mammoths but the left tusk is shorter and sloped, more like a juvenile(青少年).

    This has caused some scientists to say the Santa Rose skull may belong to a transitional species. Whatever's the case, a following examination of the mammoth's teeth should put the matter to rest. The analysis will also tell us how old the mammoth was when it died, so we can tell for sure whether it was an adult or juvenile.

    More interesting than the mammoth's lineage, however, might be its story. The giant mammal lived 13,000 years ago or roughly the same timeline of the “Arlington Man”, a 13,000-year-old human skeleton also found on Santa Rosa. Some 3,000 years later humans were already spread throughout the continent and the Channel Islands' mammoth went extinct. The present finding might help reveal a link between the two.

    The remains also seem to confirm a long-held assumption that there were two mammoth migrations to the Channel Islands. “The discovery of this mammoth skull increases the probability that there were at least two migrations of Columbian mammoths to the island: during the most recent ice age 10,000 to 30,000 years ago, as well as the previous ice age that occurred about 150,000 years ago,” said USGS geologist Dan Muhs.

    Finding out what kind and how much physical activity you need

    How do I do it?

    It's your choice. Pick an activity that's easy to fit into your life. Do at least 10 minutes of physical activity at a time. Choose aerobic activities that work for you. These make your heart beat faster and can make your heart, lungs, and blood vessels fitter. Also, do strengthening activities which make your muscles do more work than usual.

    Why should I be physically active?

    Physical activity can make you feel stronger and more alive. It is a fun way to be with your family or friends. It also helps you improve your health.

    How many times a week should I keep physically active?

    It is up to you, but it is better to spread your activity throughout the week and to be active at least 3 days a week.

    How do I build up more physical activity?

    Do a little more each time. Once you feel comfortable, do it more often. Then you can trade activities at a moderate level for vigorous ones that take more effort. You can do moderate and vigorous activities in the same week.

    Every morning, Claire Buckley was rushing to catch the bus. She left for Barrington High School in Illinois at 6 a.m.. She would come home very tired. Kids like Clair Buckley are suffering from lack of sleep. Less sleep causes stress. Stress is when someone is under a lot of pressure. Not sleeping is just one reason experts say more students feel nervous at school. Local schools in Illinois are trying new way to fix it, wishing kids to feel healthy and happy.

    Some schools are changing their calendar(日历). They are giving tests before school breaks. This way students do not have to worry about studying when they are away from school. Some places changed their after-school activities. Coaches can only organize one practice a day. Teachers now cannot give homework during certain holiday breaks. But some kids want very much to do well on tests. They think it will get them into good colleges, which causes them stress.

    A study showed different things stress different students out. Students were not able to see their grades online. This stress some students. Other students said seeing their grades made them more stressed. Some say parents are causing stress. They try to make their kids do not fail. They want their kids to not be disappointed. Children need to fail, says Dr. Carol Weitzman, a doctor for children. When kids learn to fail, they learn to get back on their feet. This is an important skills for life, she says.

    Various ways are adopted to reduce stress. Students at John Hersey High School drop by Dr. Brigette Muck's office looking for stress relief. She has aromatherapy (芳香疗法) products. The smells make you fell relaxed. She also has stress balls and coloring books to help work out nervous feelings. Other schools use mentorship(导师制) program, sports and yoga classes to help students feel better.

Help Hermit Crabs

    Hermit crabs are nature's recyclers. They eat waste and help keep oceans and shores clean. Unlike other crabs, the hermit crab has a thin outer shell over its soft tail. This makes the hermit crab easy prey for hungry predators(天敌).Hermit crabs stay safe by living in old seashells. A hermit crab is picky(狡猾); it tries on many shells until it finds one that fits just right. In recent years, however, many hermit crabs have had trouble finding their perfect homes. What is the problems. There are not enough shells to go around!

    One reason for the seashell shortage is that ocean water is not as clean as it once was. This has caused chemical changes to sea water. Some sea animals, like snails, are affected by these changes.  Now there are fewer snails making shells. People visiting the beach often take shells home as souvenirs(纪念品).This is another problem. Other people even take shells for their own pet hermit crabs! They do not realize that hermit crabs in the wild need those shells too.

    Now people are working to solve this hermit crab housing shortage. They are teaching beach-Boers to leave seashells where they belong--at the seashore! Some people even make fake seashells that they hope the hermit crabs will like. For example, a group called Project Shelter invited people who visited their website to create different designs for hermit crab shells.

    These designers had a lot to consider. What kind of material should be used to build a seashells. The material must be light enough for the hermit crab to carry, but strong enough to protect the crab from predators. The man-made shells could not contain glue or any other substance(物质)that might harm a hermit crab. Another challenge with building a hermit crab home was the opening to the shell. Too big would mean the crab would not feel safe. Too small would be uncomfortable, and the crab would not want to move in.

    The Project Shelter shells are placed in the wild for hermit crabs to find. Lucky hermit crabs can move into these new dream homes.

    Have you ever spent an afternoon in the backyard, maybe grilling or enjoying a basketball game, when suddenly you notice that everything goes quiet? There is an old phrase "calm before the storm", often used in a situation—a quiet period just before a great activity or excitement. According to our own experience, we know there is actually calm before the storm. But what causes this calm? And is it always calm before the storm? Let's hear what scientists have to say.

    A period of calm happens in a particular kind of storm, the simplest kind of storm—a single-cell thunderstorm. In this type of thunderstorm, there is usually only one main updraft, which is warm, damp air and drawn from places near the ground. Storms need warm and damp air as fuel, so they typically draw that air in from surrounding environment. Storms can draw in the air that fit their need from all directions—even from the direction in which the storm is traveling.

    As the warm, damp air is pulled into a storm system, it leaves a low-pressure vacuum (真空) coming after. The rising air meets the cold dry air that has already existed in the storm clouds, thus the temperature of the warm, damp air drops, and the water vapour (水蒸汽) in it changes into tiny drops that are a precondition of rain. These drops accumulate and build on larger particles like dust, until they grow large enough to form raindrops.

    This warm, damp air keeps moving upwards, but it becomes cooler and drier during its trip through cloud. When it reaches the top of the cloud, the air gets spit out (被挤出) at the top. This air is sent rolling out over the big thunderclouds. From there, the air goes down. Warm and dry air is relatively stable, and once it covers a region, that air, in turn, causes the calm before a storm.

    Most thunderstorms, though, don't start with calm. That's because most are actually groups of storms with complex wind patterns. There's so much air moving up and down storm groups that the calm before the storm never happens. Instead, before the storm,让 might be really windy!

    I'd done it before, and so I had no reason to believe that this time would be any different. I was sure that when I returned home from my mission trip. As always, I'd bring back nothing more some mud on my boots. A hole or two in my jeans and, of course, a lot of great memories.

    The summer before my high school graduation, I went to West Virginia with others as volunteers to repair the homes of those in need. Arriving at our destination, my group was assigned the task of rebuilding sections of a home that had been damaged by fire. No sooner had we parked on the home's dirt driveway than we saw an excited little girl, no more than six years old, standing in the doorway of the family's temporary home. Shoeless and wearing dirty clothes and the biggest smile I'd ever seen, she yelled, "Ma, Ma, they really came!" I didn't know it then, but her name was Dakota, and four more days would pass before she'd say another word near me.

    Behind Dakota was a woman in a wheelchair — her grandmother, we'd soon learn. I also discovered that my job that week would be to help change a fire-damaged dining room into a bedroom for this little girl. Grabbing our tools, we went to work. Over the following days, I noticed Dakota peeking at us every now and then as we worked. A few times, I tried talking with her, but she remained shy and distant, always flying around us like a tiny butterfly but keeping to herself.

    By our fifth and final day, however, this was about to change.

    Before I went to work on her home on that last morning, I spoke for a moment or two with the grandmother. I was especially pleased when she told me how much Dakota loved her new room — so much, in fact, that she'd begged to sleep in it the previous night, even though it wasn't quite ready. As we talked, I noticed something I hadn't seen before — Dakota was hiding behind her grandmother.

    Cautiously, she stepped into view, and I could see that just like her clothes, her face was still dirty. But no amount of soil could hide those bright blue eyes and big smile. She was simply adorable. I wanted so much to hug her, but respecting her shyness, I kept my distance.

    Slowly, she began walking toward me. It wasn't until she was just inches away that I noticed the folded piece of paper in her tiny hand. Silently, she reached up and handed it to me. Once unfolded, I looked at the drawing she'd made with her broken crayons on the back of an old coloring book cover. It was of two girls — one much taller than the other — and they were holding hands. She told me it was supposed to be me and her, and on the bottom of the paper were three little words that instantly broke my heart. Now almost in tears, I couldn't control myself anymore — I bent down and hugged her. She hugged me, too. And for the longest time, neither of us could let go.

    By early afternoon, we finished Dakota's bedroom, and so I gladly used the rare free time to get to know my newest friend. Sitting under a tree away from the others, we shared a few apples while she told me about her life. As I listened to her stories about the struggles she and her family went through daily, I began to realize how boring various aspects of my own life were.

    I left for home early the next morning. I was returning with muddy boots and holes in my Jeans. But because of Dakota, I brought back something else, too-a greater appreciation for all or the blessings of my life. I'll never forget that barefoot little butterfly with the big smile and dirty face. I pray that she'll never forget me either.