When Chelsea stepped off the school bus on her last day of kindergarten, she handed me a construction-paper card shaped like a necktie. “This is for you, Mommy,” she said. With pink and yellow tempera paint she had filled the tie with flowers, and with black she’d placed the letters M and O over the preprinted F and A in FATHER. I understood, in that moment, that the next two and a half months were to be treasured, that my days of unfettered freedom with Chelsea would be finite. Eventually, I moved the card from my refrigerator to a box on a shelf above my desk. In the years that followed, I added more treasures to the box: a seashell that Chelsea had found at the shore, a rubber Minnie Mouse and Canadian coins from our trips to Disney World and Niagara Falls, a…