NEW YORK — There’s no question Robin Williams could charm an audience, but he could also charm an ape.

Sure, Robin Williams made us smile, but what made him smile? Tickling an ape.

It was an encounter he called mind-altering, holding hands with a gorilla named Ko ko, famous for understanding sign language, taught by her surrogate mother, Penny Patterson.

Profiled by PBS, a cover girl on National Geographic, then in 2001, Williams asked to pay a visit.

” She was very drawn to him,” Patterson said.

She took his glasses and ended up wearing them. She picked his pocket and investigated his wallet. But mostly she asked: “Tickle tickle she wants you to tickle her.”

Years later, he was still using the encounter as material, but at the time of the visit, he was quiet and respectuful.

Though they only had one visit, Koko was familiar with Williams. She had recently watched a documentary he did on dolphins and she would watch “Awakenings” over and over.

In 2001, Koko was 30 years old and sad about the death of her companion ape named Michael. She hadn’t smiled in six months until Williams tickled her fancy. Patterson says Koko knew something was amiss from listening to phone calls Monday.

“I just said Koko we’ve lost a dear friend, you remember Robin?” Patterson said.