Philadelphia Mayor Jim Kenney has a sad resting face.

To many casual observers, this is an inscrutable fact. At press conferences and ribbon-cuttings, his forced smiles barely conceal a bureaucrat quietly suffering the world around him before settling back into dour repose. It is the face of a man watching Cast Away with misty-eyed recognition, howling inaudibly from his own island no one else can see. “So sad sometimes,” a voice whispers.

But to the mayor’s closest aides, this isn’t the half of it. Their 61-year-old boss is capable of at least 30 distinct facial expressions — and they have photographic proof.

Hanging on the fridge at the mayor’s City Hall office is a crisply designed magnet featuring a few dozen cut-out Kenney heads, each smirking this way and that.

It’s called a “feelings chart.”

In addition to some of the more familiar faces (“lonely,” “angry,” “sad”) it contains cropped visages of Kenney being “mischievous” and “lovestruck.” These are mayors you don’t see every day — but we are told they’re very real.

The chart showed up in a photo in an Inquirer story last week about the mayor’s 25-year-old correspondence liaison, Chamarra McCrorey, who channels her inner Kenney every day to answer thousands of emails and letters in the her boss’ voice. Naturally, we reached out.

Turns out the chart was a parting gift from Stephanie Waters, the mayor’s former digital director, who left the office last summer.

“I looked at Jim Kenney’s expressive face for three years,” Waters wrote in an email. “One day I was looking for an image of him smiling (which wasn’t always so easy to find) and it struck me that over the course of three years, our photographer Samantha Madera and I had captured the many, many faces of Mayor Kenney.”

The feelings chart was a natural gift to leave behind. Colleagues, she figured, would get a kick out of illustrating their own workplace emotions with, well, the source of most of them.

In the professional world, such charts are used by grade school teachers and psychotherapists to help parse mental states. And though they’ve become popular as office gags, in the executive branch of the sixth largest city in the country, they serve functional purpose.

“We use it to identify how we are feeling,” said Deana Gamble, the mayor’s communications director, “as well as the mood of our boss day to day.”

Close observers know Kenney regularly has bursts of outward emotion. Some of them occasionally leak into the public sphere — like the jubilant little dance in response to a sanctuary city victory that went semi-viral last year among both his fans and critics.

The mayor, who is up for re-election this year, is apparently well aware of his elastic face. The feelings chart makes him laugh, according to his office.

The first time Kenney saw it, Gamble said, he relayed a story. His fifth grade teacher in Catholic school wrote a note to his mother on the back of a report card. The take-home missive from the nun requested Mrs. Kenney for a parent-teacher sit-down, stat.

“His mother went to his school thinking her son did something wrong,” Gamble said. The teacher said, no, he wasn’t in trouble. But the nun wanted Mrs. Kenney to know something about her young son: “He shows every thought and feeling on his face.”