DI am watching my son and hundreds of his classmates, marching in perfect order into the university football stadium（体育场）. Holding back my tears, I search for my son in the sea of people. Suddenly, there he is. I jump to my feet and start waving. Arms flying back and forth above my head, I look like a windmill（风车）, or maybe an airport worker guiding a plane. I see him smile, then laugh as he pulls his camera out of his pocket. He takes a picture of me as he waves back, and then goes to find his seat.What is it about graduations that makes them so moving? Why do most of us women cry as we watch our sons, daughters, grandchildren, nieces, and nephews march in dark robes（长袍）? We cry partly because, no matter how old our children are, and no matter how much they have achieved, in our eyes they are still the little babies we carry around the house to comfort.And, of course, we cry also because something wonderful is ending, and we miss it already. Whatever is on its way to take its place will not be the same. It has been such a gift to have my son go to college close to home. We never have to worry about whether he will be able to make the trip home for a holiday. But this was a time-limited gift. My son will attend graduate school in California. We will talk and text like crazy, and visit as often as we can. But it will not be the same.The writer cries partly because _____.