There are many mysteries in nature.
Mysteries are things we don't understand, such as how the Egyptian pyramids were built.
In fact, we are surrounded by mysteries.
What we know about the world seems like very little.
Scientists tell us that what we see in the universe is only a small part of what is there.
The rest is invisible, including dark matter and dark energy.
These two unknowns make up around 95% of the universe.
Evidence for their existence comes from their effects on what we can observe, such as the motion and positions of distant galaxies.
Yet, though we know very little, we are able to build airplanes and communicate with smartphones.
In some ways we humans are very smart, or at least we think we are.
We still need to face big issues, like climate change, and that is going to be a huge challenge.
The challenge for humans will be our ability to adapt to our changing environment.
What about other forms of life?
How do they adapt?
Seasonal variations, for example, can be one of the greatest challenges for animals to overcome.
Overcoming such challenges is a great achievement, but hardly easy.
Some birds migrate thousands of kilometers each year to deal with seasonal changes.
These migrations are amazing.
Birds don't carry maps or compasses, yet they know where to go.
They have developed the ability to see the Earth's magnetic field.
This ability is a wonder of nature.
It takes advantage of the fact that the earth is like a huge bar magnet.
It has a north pole, which is positive, and a south pole, which is negative.
Somehow the birds are able to see this magnetic field and use it to their advantage.
And even the way birds fly together is amazing.
Many migrating birds fly in a V formation.
They do this to conserve energy.
In a V formation, birds can use the energy of the birds in front of them.
This energy comes from the air currents generated by the wings.
Conserving energy is necessary for the birds to survive the long journey.
How do the birds know this?
It's truly amazing how the birds know this.
Egyptian pyramids 英 [iˈdʒɪpʃn ˈpɪrəmɪd] 美 [iˈdʒɪpʃn ˈpɪrəmɪd] 埃及金字塔
surround 英 [səˈraʊnd] 美 [səˈraʊnd] v. 围绕;环绕;(使)包围，围住;与…紧密相关,
invisible 英 [ɪnˈvɪzəbl] 美 [ɪnˈvɪzəbl] adj.看不见的;隐形的;无形的(与服务而非商品有关) n. 看不见的人
existence 英 [ɪɡˈzɪstəns] 美 [ɪɡˈzɪstəns] n.存在;实有;(尤指艰难或无聊的)生活，生活方式
distant 英 [ˈdɪstənt] 美 [ˈdɪstənt] adj. 遥远的;远处的;久远的;不相似的;不同的;远亲的;远房的
galaxy 英 [ˈɡæləksi] 美 [ˈɡæləksi] n. 星系;银河;银河系;群英;人才荟萃
variation 英 [ˌveəriˈeɪʃn] 美 [ˌveriˈeɪʃn] n. (数量、水平等的)变化，变更，变异;变异的东西;变种;变体;变奏;变奏曲
overcome 英 [ˌəʊvəˈkʌm] 美 [ˌoʊvərˈkʌm] v. 克服;解决;战胜;受到…的极大影响
achievement 英 [əˈtʃiːvmənt] 美 [əˈtʃiːvmənt] n. 成就;成绩;功绩;达到;完成
compass 英 [ˈkʌmpəs] 美 [ˈkʌmpəs] n. 罗盘;罗经;指南针;罗盘仪;圆规;两脚规;范围;范畴;界限
magnetic 英 [mæɡˈnetɪk] 美 [mæɡˈnetɪk] adj. 像磁铁的;有磁性的;磁的;磁性的;富有吸引力的;有魅力的
magnet 英 [ˈmæɡnət] 美 [ˈmæɡnət] n. 磁铁;磁石;吸铁石;有吸引力的人(或地方、事物);磁体;磁性物体
somehow 英 [ˈsʌmhaʊ] 美 [ˈsʌmhaʊ] adv. 以某种方式(或方法);由于某种未知的原因;不知为什么;不知怎么地
migrate 英 [maɪˈɡreɪt] 美 [ˈmaɪɡreɪt] v. (随季节变化)迁徙;移居;迁移;移动;转移
formation 英 [fɔːˈmeɪʃn] 美 [fɔːrˈmeɪʃn] n. 组成;形成;组成物;形成物;编队;队形
conserve 英 [kənˈsɜːv , ˈkɒnsɜːv] 美 [kənˈsɜːrv , ˈkɑːnsɜːrv] v. 节省;节约;保护;保存;保藏 n. (含有大块或整块水果的)果酱，蜜饯
