How come some insects are able to walk on the surface of a pond,
为什么有些昆虫可以在水面上轻轻拂过，
but you quickly sink to the bottom when you try to walk on water?
但是当你试图踏入水中时却立即沉入水底？
And why do lakes freeze from the top down in winter?
为什么冬天的时候湖水是从上往下结冰呢？
In a word, the answer to all these questions is polarity.
简单地说，以上所有问题的答案都是极性。
Water is a simple molecule made up of one oxygen atom and two hydrogen atoms, but it is essential to life.
水是一种简单的分子，是由一个氧原子和两个氢原子组成的，但水仍然是生命之源。
In fact, water makes up approximately 60% of the adult human being's body weight.
事实上，一个成年人60%的体重都是由水组成的。
The polarity within those water molecules gives this common substance the properties that make it unique and life-sustaining.
这些水分子的极性给予了这个简单的物质独特且能够维持生命的性质。
Polarity refers to the unequal sharing of electrons within a molecule.
极性指的是一个分子内电荷分布的不均匀性。
For water, the bonding between the oxygen atom and two hydrogen atoms within a single water molecule
对水来说，在一个水分子中，氧原子和两个氢原子之间的化学键，
is like a tug-of-war between a big, strong football player and a cute little toddler.
就像一个高大强壮的足球运动员和一个可爱的婴儿之间进行拔河。
Oxygen is a larger atom, with more protons in its nucleus than hydrogen.
氧原子更大一些，它的原子核中比氢原子有更多的质子。
These positive charges are like a person's physical strength.
这些正电荷就像一个人的力气一样，
They're able to attract the negatively charged electrons in the bond,
它们能够吸引带负电荷的电子，
just like a strong individual is able to overpower a weaker individual in a tug of war.
就像较强的一方可以在拔河比赛中战胜较弱的一方。
So, oxygen is able to attract more than its fair share of electrons.
所以，氧原子能够比氢原子吸引更多的电子。
Because hydrogen is smaller and has less strength, or fewer protons,
因为氢原子体积小，有更小的力量，更少的质子，
it loses the tug of war and attracts fewer than its fair share of electrons.
它在拔河比赛中输了，于是就比氧原子吸引更少的电子。
So, the oxygen in water behaves as though it's negative, and the hydrogens behave as though they're positive.
因此，水当中的氧原子带负电，氢原子带正电。
The bonds within a single water molecule are called polar covalent bonds.
一个水分子中的化学键叫做极性共价键。
Covalent means that the electrons are shared.
共价代表电子由两个原子共有。
But, as we just learned, polar means that these electrons are not shared equally.
但是，正如我们刚才所学的，极性表示电子不是均匀分布的。
In water, the oxygen acts negative and the hydrogens act positive.
在水分子中，氧原子是带负电的，而氢原子是带正电的。

Since negative and positive attract,
由于异性相吸，
that oxygen is attracted to the hydrogen atoms in neighboring water molecules.
所以氧原子和氢原子在不同的水分子之间相互吸引。
A special type of bond forms between water molecules, known as a hydrogen bond.
水分子之间产生了一种特殊的化学键，叫做氢键。
Hydrogen bonds don't just happen in water either.
氢键不只是在水分子之间存在。
They can form between a water molecule and different substances that are polar or ionic.
他们也可以在水分子和不同的极性分子或者离子化合物之间存在。
Water's ability to stick to itself is called cohesion,
水分子能够簇在一起，这叫做凝聚力，
while water's ability to stick to other substances is called adhesion.
水能够粘在别的物质上，这叫做黏附性。
Now, think back to the initial questions.
现在，再回想一下最一开始的问题。
First, why are some insects able to walk on water?
第一，为什么有些昆虫能够在水上爬行？
Surface tension due to hydrogen bonding creates a thin film on the surface of water
由于氢键，表面张力在水面上产生一层薄膜
that gives enough resistance for super-light insects to walk on.
它的弹性足以支撑小型昆虫在上面走动。
You can't walk on it because the hydrogen bonds aren't strong enough to hold you up.
而你不行，是因为氢键的强度不足以支撑你。
Why does ice float on top of liquid water?
为什么冰能够漂浮在液态水中呢？
For most other substances, the solid state is more dense than the liquid state, but that is not the case for water!
对于大多数物质来说，固态的密度总是比液态的密度大，但是水却不是这样！
Hydrogen bonds keep water molecules farther apart in frozen water than in liquid water.
在冻结的水中，氢键使得水分子之间的距离要比在液态水中更大，
The farther apart the molecules are, the less dense that solid is.
分子之间的距离越大，固体的密度就越小。
So ice is about 9% less dense than water, which means it floats on the top.
所以冰的密度比水大约小9%，这就使得它能漂浮在水上。
That's why lakes freeze from the top down and aquatic life is able to survive through a cold winter every year.
这就是为什么湖水是从上往下结冰，而水生生物能够度过每年寒冷的冬天。
It is the polarity of the water molecule and the resulting hydrogen bonding that account for water's unique properties.
水分子的极性和它造成的氢键构成了水独特的性质。
So, the reason that water is so special,
因此，水是如此特殊的原因，
from inside your cells to the world's oceans, is simply because it is a polar molecule.
从你身体中的细胞到地球上的海洋，都是因为它是一个极性分子。