“In digging things out of the ground—which you probably do now and then,” the Professor began in a loud voice; “in making things into heaps—which no doubt you often do; and in kicking things about with one heel—which you seem never to leave off doing; have you ever happened to notice another Professor, something like me, but different?” “Never!” shouted the Gardener, so loudly and violently that we all drew back in alarm. “There ain’t such a thing!” “We will try a less exciting topic,” the Professor mildly remarked to the children. “You were asking——” “We asked him to let us through the garden-door,” said Sylvie: “but he wouldn’t: but perhaps he would for _you_!” The Professor put the request, very humbly and courteously. “I wouldn’t mind letting _you_ out,” said the Gardener. “But I mustn’t open the door for _children_. D’you think I’d disobey the _Rules_? Not for one-and-sixpence!” The Professor cautiously produced a couple of shillings. “That’ll do it!” the Gardener shouted, as he hurled the watering-can across the flower-bed, and produced a handful of keys—one large one, and a number of small ones. “But look here, Professor dear!” whispered Sylvie. “He needn’t open the door for _us_, at all. We can go out with _you_.” “True, dear child!” the Professor thankfully replied, as he replaced the coins in his pocket. “That saves two shillings!” And he took the children’s hands, that they might all go out together when the door was opened. This, however, did not seem a very likely event, though the Gardener patiently tried all the small keys, over and over again. At last the Professor ventured on a gentle suggestion. “Why not try the _large_ one? I have often observed that a door unlocks _much_ more nicely with its _own_ key.” The very first trial of the large key proved a success: the Gardener opened the door, and held out his hand for the money. The Professor shook his head. “You are acting by _Rule_,” he explained, “in opening the door for _me_. And now it’s open, we are going out by _Rule_—the Rule of _Three_.” The Gardener looked puzzled, and let us go out; but, as he locked the door behind us, we heard him singing thoughtfully to himself “He thought he saw a Garden-Door That opened with a key: He looked again, and found it was A Double Rule of Three: ‘And all its mystery,’ he said, ‘Is clear as day to me!’” “I shall now return,” said the Professor, when we had walked a few yards: “you see, it’s impossible to read _here_, for all my books are in the house.” But the children still kept fast hold of his hands. “_Do_ come with us!” Sylvie entreated with tears in her eyes.