See if you can spot what's wrong in these photos of crowds

You can look at a Pelle Cass photo for several moments before realizing it doesn't make sense.

See if you can spot what's going on in this picture of people walking down the stairs:

Figured it out?

"Each person's foot is hovering an inch or so above the next step," Cass says by email. "The odds that 19 strangers would be caught at the same crucial instant in the same instantaneous photograph just before landing on the next step must be astronomically small."

How does Cass do it? Calling himself a subversive trick photographer, the Boston artist takes hundreds of photos on a tripod in a single spot over about an hour. He then goes back to his studio and carefully selects content to include in a composite image.

"I don't change a thing and I never move a figure or doctor a single Pixel," he explains. "I simply decide what stays in and what's left out."

Photos in "Selected People" can show a perfect spectrum of colors, a collection of people raising their arms, or simply an arrangement the artist finds striking.

"I never pass up the chance to make a joke, visual or otherwise," he adds.

Cass shared a set of photos from "Selected People," including a few never seen before. See if you can spot what's wrong.