Zizhen, due to all the romance books he's read, is a little convinced that he would know the exact moment he fell in love. Instead, he's already fallen in love by the time he realizes that the warmth and happiness and need to be close that he's feeling are parts of the love he has for Lan Sizhui. Zizhen would realize in a quiet moment. Maybe Lan Sizhui smiled wider, maybe he leaned so their shoulders touched, maybe he was even just sitting next to Zizhen. Whatever the exact moment, Zizhen looked at Lan Sizhui and went 'oh' as he realized he had fallen in love without even fully realizing it.