七桥问题，为什么一次走不完？

A city once had seven bridges over a river. 从前有座城市，河上有七座桥。 People wanted to walk across all seven. 大家想把七座桥都走一遍。 They wanted to use each bridge only once. 而且每座桥只想走一次。 No matter how they tried, it did not work. 不管怎么走，都走不下来。 Euler proved it was impossible. 数学家欧拉证明了这不可能。 His idea started a whole new kind of math. 他的思路开创了一门新数学。 Today we call it graph theory. 这门数学现在叫图论。