叶落知秋[ yè luò zhī qiū ]

⒈ 见树叶纷纷落下，就知道秋天到了。比喻从事物发展的微小变化可以预测事物的发展。例落叶知秋，举一明三。——宋·普济《五灯会元》英the falling leaves announce the approach of autumn—revealing signs foretell things to come;