题面摘自 Henry E. Dudeney 公版文本；英文为古腾堡原文整理，中文为本站自译，提示、解答骨架和闲谈保留本站原创结构。
The precocity of some youths is surprising. One is disposed to say on occasion, "That boy of yours is a genius, and he is certain to do great things when he grows up;" but past experience has taught us that he invariably becomes quite an ordinary citizen. It is so often the case, on the contrary, that the dull boy becomes a great man. You never can tell. Nature loves to present to us these queer paradoxes. It is well known that those wonderful "lightning calculators," who now and again surprise the world by their feats, lose all their mysterious powers directly they are taught the elementary rules of arithmetic. A boy who was demolishing a choice banana was approached by a young friend, who, regarding him with envious eyes, asked, "How much did you pay for that banana, Fred?" The prompt answer was quite remarkable in its way: "The man what I bought it of receives just half as many sixpences for sixteen dozen dozen bananas as he gives bananas for a fiver." Now, how long will it take the reader to say correctly just how much Fred paid for his rare and refreshing fruit?
一些年轻人的早熟令人惊讶。有时人们会说：“你这个孩子是个天才，长大后一定会做出伟大的事情。”但过去的经验告诉我们，他总是成为一个相当普通的公民。相反，常常出现这样的情况：一个愚钝的男孩变成了一个伟人。你永远无法判断。大自然喜欢向我们展示这些奇怪的悖论。众所周知，那些神奇的“闪电计算器”，他们时不时地用他们的功绩让世界惊叹不已，一旦他们学会了基本的算术规则，他们就会失去所有神秘的力量。一个男孩正在拆一根精选的香蕉，一位年轻的朋友走近他，用羡慕的目光看着他，问道：“弗雷德，你买那根香蕉花了多少钱？”迅速的回答是相当引人注目的：“我买的那个人用六便士买十六打香蕉，只得到他卖五块香蕉的一半。”现在，读者需要多长时间才能正确说出弗雷德为他的稀有清爽水果花了多少钱？
提示 1
先只保留连接关系，把多余长度和角度擦掉。
提示 2
数端点、奇偶穿越和连通块，看看有没有不变量。
提示 3
最后再把抽象关系放回原图，确认没有偷换路径。
完整解答
一笔画回到起点需要所有路口度数为偶数；若有四个奇度路口，就不可能每条路恰好走一次并回到原点。至少要改变两条端点关系，或增加一条连接两个奇度点的路，把奇度点数降到 0。
Dudeney 的趣题常把难点藏在“看起来可以试”的地方。别急着猜答案；先把图、表或状态画出来，再问哪些限制一直没有变。这也是它和 Carroll 逻辑题互补的地方：一个拆句子，一个拆结构。