"They're not gonna want me to play 'babies in space'," says Greg O'Brien. "You know, where I pick 'em up in my hands and I swirl them around over my head like a rocket ship. I always say 'Babies! In! Spaaaaace!' "It's October 2016, and he is musing about the latest O'Brien family news. His daughter, Colleen, is due to have a baby in November, and ever since he found out, Greg has been struggling with competing emotions."I'm not quite sure what to expect," he says. "Am I excited? God, yes. And will it lift me up? Yes it will." Eight years after he was diagnosed with early onset Alzheimer's disease, O'Brien knows that becoming a grandfather could add new motivation and joy to his life.But eight years of memory loss, depression, anger and mental decline have left 67-year-old O'Brien, who lives in Brewster, Mass., a different man than the outgoing writer and father who raised Colleen. He is easily confused, overwhelmed by groups and exhausted by conversation.Sometimes he is paranoid. Sometimes he knows he is paranoid.So even before his grandchild arrives, he knows there will be physical limits on how he can interact with the infant. "You know, they're not gonna want me to put the baby on my shoulders. Hug the baby, pat the baby," he says. "I get it. I'm not going to fight it."There are other problems, too, with meeting a new person when your mind is in decline. Greg's identity is slipping and shifting under him. He never used to be angry or anxious. "You know, I'm an imperfect guy, but an OK guy," he says, "and my grandchildren really aren't going to get to know me. Someone else will have to tell them.""I feel a little bit, when I think about grandkids, like a shell of myself. So it's kind of bittersweet.""Maybe through my writing, through other things, you leave a legacy behind," he says. "I may try to connect in the heart and the soul. Just looking, smiling, touching and have my grandkids remember me that way."AdelineIt's Christmas. Adeline is 1 month old, and she spends most of her time sleeping. The house on Cape Cod is full of people, celebrating and talking. Colleen sees her father leave a crowded living room, seeking quiet and solitude. She finds him later sitting with his granddaughter, both of them content in their own worlds.It's January. In Massachusetts, the snow is 2 feet deep. The Christmas tree made it out of the living room and onto the back porch, only to be stranded in a snowdrift.When Greg sees it out of the corner of his eye, his mind transforms it into a billowy monster, crouching outside the door.But it's just a tree, and he pulls his mind back to reality and snaps a picture on his phone. Two-month-old Adeline would like this tree monster. He sends her a text, c/o her mother.