When you pay to enter a museum, they usually give you something like a wrist band or a sticker. My favorite proof-of-admission is the little metal button at the Metropolitan Museum of Art. Tall boxy bins flank the museum's exit doors. Visitors remove their buttons and toss them in the bins as they leave. The buttons change color each day, so the clear plastic bins hold layers of color, like a large-scale version of colored sand in a bottle. At the Cooper Hewitt, a slab of clear Plexiglas stands near the door. You can place your sticker on the slab. I love this! It's art that requires the participation of gallery visitors. Each sticker has traveled through the museum. Each sticker represents an experience. And the gathering of stickers just looks really cool.