從前有個綽號叫張鐵頭的農夫，這是因為他忠厚老實、反應遲鈍不曉得變通的緣故。他每天勤勞的種田，可是連年碰到颱風、水災或旱災，收成一年不如一年，有一天，家裡突然需要一筆錢，太太叫他去羅財主家借錢應急，等收成時再想辦法還錢。
張鐵頭只好硬著頭皮去借錢，到的時候，羅財主正好在吃甘薯，看到他來便叫他一起吃，以前人生活清苦，一般人都是把甘薯曬成甘薯籤收起來，很少能吃到整顆的甘薯，羅財主太有錢，所以是把皮剝掉再沾糖吃，這是很奢侈的事，張鐵頭拿到甘薯時，心想：「應該連皮都吃掉，這樣才不會浪費，可是，人家羅財主是把皮剝掉才吃，我應該要學他吃，免得被人笑我是沒見過世面的鄉下人。」
於是，張鐵頭把皮剝掉開始吃甘薯，等吃完後，才向羅財主表明是來借錢救急的，羅財主低頭想了一會兒說：「你想借的錢對我來說是一筆小數目，我本來可以借你的，可是剛才看你吃甘薯，把皮給剝掉，一點都不珍惜，可見你平時是個很浪費的人，我不想借錢給不懂得節省的人。」憨厚的張鐵頭聽到後，完全無法辯解，只好摸摸頭回家。
老婆知道後，搖頭說：「你這個人的腦袋真是不知變通，『吃甘薯也不知道看勢面』啊！」後來人們就把這句話用來提醒人在做事時，要懂得靈活變通。
There was once a farmer who was nicknamed Iron Head Chang. This was because he was an honest man, but he was slow and inflexible when dealing with problems. He worked hard in the fields every day, but year after year of typhoons, floods, and drought caused his harvest to shrink year by year. One day, his family needed money urgently. So, his wife asked him to go to the millionaire Lo to borrow some money, and they would repay the money after the harvest.
Iron Head Chang had no choice but to pay a visit to the millionaire Lo. When he arrived at Lo's house, Lo happened to be eating sweet potatoes, and invited Chang to have some as well. Back in those days, life was very tough, and few people could afford to eat sweet potatoes whole. They had to dry the sweet potatoes under the sun and cut them into thin strips, which they ate sparingly. As Lo was very rich, he could afford to peel the skin off the sweet potato and dip it in sugar before eating it. This was considered to be a great luxury. When Chang was offered a sweet potato, he thought to himself, “I should eat the skin to avoid wasting food, but Lo peeled the skin off before eating it. I should do the same so that people won't think of me as a country bumpkin.”
In the end, Iron Head Chang decided to eat the sweet potato in the same way as the millionaire Lo. After eating, Lo asked Chang why he had come, and Chang told him he needed to borrow some money. Lo thought for a while and said, “The amount you wish to borrow is a trifling sum to me. I would have lent it to you, but seeing the wasteful way in which you ate the sweet potato just now, I can tell that you are a wasteful person by nature. I do not wish to lend money to someone who does not know the value of thriftiness.” When Iron Head Chang heard this, he could think of nothing to say, so he went home empty-handed.
After his wife heard what happened, she sighed and said, “You are so inflexible. You should have eaten the sweet potato in a manner that befits your means and circumstance!” Later on, people used this saying to remind themselves of the value of being flexible, and to behave in a manner that befits their means and circumstance.
