One day he learned, quite by accident, a new and very useful way of helping his people. He and A Hoa and several other young men who had become Christians, went on a missionary tour to Tek-chham, a large city which he had visited once before. On the day they left the place, Kai Boksu's preaching had drawn such crowds that the authorities of the city became afraid of him. And when the little party left, a dozen soldiers were sent to follow the dangerous barbarian and his students and see that they did not bewitch the people on the road. The soldiers tramped along after the missionary party, and with his usual ability to make use of any situation, Mackay stepped back and chatted with his spies. He found one poor fellow in agony with the toothache. This malady was very common in north Formosa, partly owing to the habit of chewing the betel-nut. He examined the aching tooth and found it badly decayed. "There is a worm in it," the soldier said, for the Formosan doctors had taught the people this was the cause of toothache. Mackay had no forceps, but he knew how to pull a tooth, and he was not the sort to be daunted by the lack of tools. He got a piece of hard wood, whittled it into shape and with it pried out the tooth. The relief from pain was so great that the soldier almost wept for joy and overwhelmed the tooth-puller with gratitude. And for the remainder of the journey the guards sent to spy on the missionary's doings were his warmest friends. After this, dentistry became a part of this many-sided missionary's work. He went to a native blacksmith and had a pair of forceps hammered out of iron. It was a rather clumsy instrument, but it proved of great value, and later he sent for a complete set of the best instruments made in New York. So with forceps in one hand and the Bible in the other, Mackay found himself doubly equipped. Every second person seemed to be suffering from toothache, and when the pain was relieved by the missionary, the patient was in a state of mind to receive his teaching kindly. The cruel methods by which the native doctors extracted teeth often caused more suffering than the toothache, and sometimes even resulted in death through blood-poisoning. A Hoa and some of the other young converts learned from their teacher how to pull a tooth, and they, too, became experts in the art.