"I saw her," Christine McKinney told us, "and I thought, 'You're an Oscar winner—what are you doing in a box?"

McKinney was standing a few feet from the glass-and-steel box where actress Tilda Swinton was sleeping for her 1995 piece, "The Maybe," at the Museum of Modern Art. Swinton's performance art was an unexpected treat and conversation piece for the museum's visitors yesterday. "The Maybe" involves Swinton lying in the glass box on top of a mattress, with just her glasses and a carafe of water, for the MoMA's operating hours, which were 10:30 a.m. until 5:30 p.m. on Saturday. (Update: Swinton is at the MoMA on Monday, March 25, for another performance!)

McKinny was visiting from Ohio—she was accompanying a youth choir on a trip to New York—and said that their group was at the museum because they had time before seeing a Broadway musical (Cinderella). She recognized Swinton immediately, though she couldn't remember that Michael Clayton was the film she won an Oscar for—but then McKinney's instincts as a nurse kicked in: "I'm wondering is she medicated... doesn't she need to go to the bathroom—is there a pad?—and did she dehydrate herself beforehand... I'm also thinking about blood clots." In spite of all that, she was still very excited to see her.

Concern for Swinton was an overwhelming theme—many museum visitors kept wondering how she would go to the bathroom—as was admiration for how rigorous it must be staying prone in the box all day. But Michele Keller, of New York, also wondered if she was medicated. She felt "confused" and unsure how she should feel about the work because there was no artist's statement. But Vincent Martinez, of Queens, thought it was wonderful, noting how it was "so bold" for her to put herself out there like that.

"The Maybe," part of the MoMA's effort to share "historic" performance art, will be staged about six more times this year—each time "unannounced and in a different location in the Museum." Yesterday's staging was a serendipitous boon to those enjoying other parts of the institution. Pablo Johannsson and Claire Cutting had just seen the Henri Labrouste exhibit when they saw the crowd around the box—Johannson thought Swinton was "brilliant" and noted how difficult and physically challenging the performance must be. He added that it made sense for her to do this, since she does edgier films.

New Yorker Helmut, who had taken his family to see the Invention Abstraction, thought "The Maybe" was interesting—and he knew recognized Tilda Swinton in the box. His 13-year-old son said, "I knew it was her immediately"—a The Chronicles of Narnia fan, we gather!

Emma, of New York, had no idea it was a famous actress in the box. In fact, she and her friends from New Orleans didn't realize it was a live person until Swinton turned. Observing "The Maybe" from the second floor, Emma said, "She looks very serene. She looks like my mother napping." But she admitted, "I thought it was a dude."