Why does ice float if it has not become lighter?
冰块没变轻，为什么浮在水上？
杯子里的冰块，常常浮在水上。
Ice cubes in a glass often float in the water.
它明明是水变的，为什么没有沉下去？
They are made from water, so why do they not sink?
只说“冰比较轻”，还没有把事情说清楚。
Simply saying “ice is lighter” does not explain everything.
我们先看同一份水，没有流走，也没有再加水。
First, consider the same amount of water, with none lost or added.
在普通情况下，这份水变成冰后，重量并没有变少。
Under ordinary conditions, that water does not lose weight when it becomes ice.
变化的是它占的地方：变成冰后，它占的地方更多。
What changes is the space it takes up: as ice, it occupies more space.
水是由很小的水分子组成的，我们用眼睛看不到它们。
Water consists of tiny water molecules that we cannot see with our eyes.
变成普通的冰时，水分子之间留出的空隙更大了。
In ordinary ice, the gaps between water molecules become larger.
水没有少，却需要更大的地方来放。
There is no less water, but it needs more room.
再换一种比较方法：看同样大的冰和水。
Now compare them differently: look at equal volumes of ice and water.
这时，冰比水轻，因为同样大的地方里，冰含的水分子更少。
Then ice is lighter than water, because an equal volume of ice contains fewer water molecules.
冰比同样大的水轻，所以普通的冰能浮在水里。
Ice is lighter than an equal volume of water, which is why ordinary ice can float in water.
浮着的冰也不是全部在水面上，通常大部分还在水下面。
Floating ice is not entirely above the surface; most of it is usually underwater.
原来，变成冰没有让那份水变轻，而是让它占了更多地方。
Freezing has not made that amount of water lighter; it has made it take up more space.
