My balls, when I’m not using them. (The brass ones are kept in a climate-controlled vault). People will occasionally pick them up and play with them.
The reason why I have them in my office: several years ago, a colleague arranged to have a construction crew replace a telescope dome with a new, sexy radar dome (transparent in all the right places, er, frequencies). The crew cut up the old dome and put it in a dumpster for disposal. Unfortunately, it was a construction dumpster that belonged to another contractor, and they refused to pay to have it emptied, and it became a big mess which I dubbed “Dumpstergate,” (I’m so frikkin’ clever sometimes) and it dragged on and on. It took a long time to find a way to pay to have the dumpster emptied without running afoul of arcane government spending protocols. This lingered to the point where the colleague retired, so another colleague put together a gag gift of a mini-dumpster-truck, and needed a ping-pong ball to slice up and represent the telescope dome. I bought a pack of six, gave him two, gave two to another, and kept two for myself.