Sun Yuan Zhou finds a lighter but it can only be used before a box is opened. Lan Zhan tests Yuan Zhou to see if he is the informant. Yi Xie uses his keen sense of smell to open a box and finds a pair of scissors. His smell can only be used once a day. Lan Zhu tests the remaining players and then declares there is an informant among them. They surmise the informant must have been the first person to enter the Door. (Source: frankly at MyDramaList)