Here is a  map of a country in which the circles represent towns and the dotted lines the railways connecting them. 
A man lived in the town marked A .
He was who was born there, and during the whole of his life had never once left his native place. 
From his youth upwards he had been very industrious, sticking incessantly to his trade, and had no desire whatever to roam abroad. However, on attaining his fiftieth birthday he decided to see something of his country, and especially to pay a visit to a very old friend living at the town marked Z. 
He would start from his home, enter every town once and only once, and finish his journey at Z. 
As he made up his mind to perform this grand tour by rail only, he found it rather a puzzle to work out his route, but he at length succeeded in doing so. 
How did he manage it? 
Do not forget that every town has to be visited once, and not more than once.