Koko remembered Robin Williams for thirteen years.

They had spent one afternoon together at the Gorilla Foundation in Woodside, California, in 2001. He had been invited to meet her because she had been in a long quiet. Six months earlier her childhood companion Michael had died, and her caretakers said she had not smiled since.

Williams was a man who had spent his whole career making other people laugh, and most of his own life trying to outrun a private darkness. He arrived expecting to entertain her. But she had other ideas.

She pulled him to the floor. The glasses came off his face. She put them on hers. Then she found his wallet in his pocket and went through it carefully, one card at a time. He tickled her. She tickled him back. At some point during all of this, she smiled.

But what is less remembered is what happened to him.

Penny Patterson, who had raised Koko and worked with her for thirty years, said that Williams seemed transformed by the visit. The high-energy entertainer who had walked in went somewhere else for a few hours. He sat with a grieving gorilla on the floor and slowed all the way down. By the time he left, he was mellower and quieter than the man who had arrived. He told an interviewer afterward that it had been a mind-altering experience and that what they had shared was extraordinary. He used the word laughter, but he meant something more particular than that. He had spent his whole career performing laughter for strangers. This was the other kind.

Thirteen years later, in August 2014, Patterson was on the phone in another room at the Foundation. Koko was nearby. The news was breaking. Patterson said gently into the phone that they had lost a dear friend, Robin Williams.

Koko went quiet. She lowered her head, and her lip began to quiver. She had met him once, for one afternoon, more than a decade earlier. She remembered.

What was exchanged that afternoon was bigger than a smile that broke a long quiet. Two beings, each with their own private grief, met each other, recognized something, and both went away changed. One of them carried the memory for the rest of her life. The other did not have thirteen more years. But he had the afternoon.