It was a period in sculpture of earthworks, specific objects and conceptual art. Michael Asher, for example, would show a trailer which each day was parked in a different location. After several visits, to the University, the zoo and other points of interest in Münster, we began thinking about a subject in separate parts which together would form a whole. Coosje, recalling that a former Bishop of Münster, known as "Bombing Berend", had laid siege to her birthplace, Groningen, in 1672, suggested cannonballs, scattered about the city. We had seen such a ball embedded in an ancient wall in Münster, and were also aware of the bombing attack on the city during World War II, which had obliterated the city’s center, now restored to its original appearance.
We began by sticking round labels on photographs of various city sites and shading them to look like balls. On a panoramic aerial view, giant "balls" were shown not only filling the city but the entire countryside beyond it. What the concept lacked was a connection to the ordinary life of the present. Coosje brought up an even more extreme proposal by Claes for Central Park, New York, in 1967, in which all the vegetation was removed and the park visualized as a pool table. Pool balls the size of skyscrapers moved at a barely visible pace around the leveled park area. This became the link we were looking for: the historical cannonball subject became the contemporary Pool Balls. We set off to study the possibilities in a local pool hall.
The Pool Balls were cast in concrete, in halves that were joined at the site. Like our friend Donald Judd, who also had a concrete work in the show, we chose not to paint the sculpture. The Pool Balls had no colors, no numbers; there was only the green of the grass. Soon after the installation, citizens began covering the Balls with images and graffiti; the Balls became a kind of message center, a bulletin board, as well as a perch for watching Rock concerts.