Suppose I am a plumber, and I take a section of pipe, insert a blowgun dart, and blow. I just shot a dart out of a blowgun. When did the pipe turn into a blowgun, though?
Did it happen when I formed the intention to use the pipe as a blowgun? No: I do not have the power to make new material objects come into existence just by thinking about it.
When I picked up the pipe? There are at least there is contact. But surely it's not the right kind of contact. It would be magic if I could make a new material object come into existence by just picking up a material object with a certain thought in mind.
When I inserted the dart? Presumably, not any insertion will do, but one with a plan to blow. For I could just be doing plumbing, using the outer diameter of the dart to measure the inner diameter of the pipe, and that shouldn't turn the pipe into a dart. Again, we have some magic here--thinking about the pipe in one way while inserting the dart creates a blowgun while thinking about it another way leaves it a boring pipe. Moreover, putting the dart into the pipe seems to be an instance of loading a blowgun rather than making a blowgun.
When I fired the dart? Surely, that's too late. As I lift up the pipe and point it at the target, I am surely pointing a blowgun!
Further complication: I now put the blowgun down among the pipes on the back of my truck, and next day install it in Mr. Smith's sprinkler system. Does Mr. Smith now come to be a blowgun owner, with the rights, liabilities and responsibilities attendant on having such a weapon (blowguns are illegal in California or Canada, after all)? Moreover, did I violate my contract with Mr. Smith, for I agreed to install pipes, whereas I installed a blowgun instead? The last question is perhaps not so pressing--for perhaps the tubularly arranged matter I installed in his garden constitutes both a pipe and a blowgun.
One might think some of the difficulties could be removed by saying that throughout I was dealing with one object, a pipe, which acquired an extrinsic property, being a blowgun. There need be no magic when a material object acquires an extrinsic property as a result of my thinking. When I think about your car, your car acquires the property of being thought about by me. But this is mistaken. Suppose I add sights to the blowgun. The sights come to be a part of the blowgun, but they do not come to be a part of the pipe--they are, rather, attached to the pipe. So the blowgun seems to be a material object distinct from the pipe.
The solution to all this is to deny that there are pipes and blowguns. There is just matter (or fields) arranged pipewise and blowgunwise. And for convenience we adopt ways of speaking that make it sound like such objects are among the furniture of the universe.