A marathon is about 42 kilometers long.
马拉松全长约42公里。
It is exactly 42.195 kilometers.
准确数字是42.195公里。
Long ago, the distance changed often.
早年的马拉松，距离并不固定。
In 1908, the games were in London.
1908年，奥运会在伦敦举办。
The Queen wanted to watch from her house.
王后想从自己家看全程。
So the start was moved back a little.
于是起点往后挪了一小段。
That number became the rule forever.
这个数字，从此就成了规矩。
