The Plump and the Slender

Once, Cheng Mo left her a message on her profile:

Winter hasn't left yet, but spring is already here.

Looking at the brief eleven characters on the screen, she stared at them for a long, long time, then wrote exactly eleven characters in reply:

Spring is here, but winter has yet to leave.

In those years, Cheng Mo and she were far apart, one in the south and the other in the north.