Spring doesn't end and then turn into summer, and autumn doesn't wait for summer to finish. Spring is already bringing on the summer air. Autumn is already there inside summer. Autumn gets cold right away, and then in early winter there are mild, sunny days, the grass turns green again and the plum trees bud. Leaves don't fall first and then the new buds come. The buds are already swelling underneath, the old leaves can't hold on any longer, and they fall. The new thing is ready and waiting below, so the change comes very fast.