The Bucket Piece

At a popular contemporary art gallery in a large city, in a small white room, place a nondescript bucket on a centered pedestal with a single light above it. A guard is stationed outside, allowing only one person in at a time. Next to the bucket is a plaque reading:

At the end of the day, any money collected in this bucket will be given to the artist. You are free to add or remove money as you wish.

This is the most elegant manifestation of the piece, but a better manifestation will get others involved. For example, donating the money to a charity instead of the artist. Or multiple buckets going to different causes. Or some kind of system measuring the contributions in real time and reporting how many children have been saved from starvation for another day.