Pixies, perhaps?

This little mystery has had me baffled for the past week or so. I've examined the situation from every conceivable angle, and still haven't been able to come up with a tenable theory. Maybe you can help?

One morning last week, I was getting ready for work. One of the last steps in this process, prior to walking out the door, is putting on my work shoes. On this particular morning, I had put my shoes on, tied the laces, and was almost ready to depart when I felt something inside my right shoe. Upon removing the shoe, I discovered a tiny stone.

Here beginneth the mystery.

I'm on my feet just about the entire day, so I notice fairly quickly if a foreign body has taken up residence inside my footwear. At no time during the previous day had I felt anything, nor was there anything in my shoe when I took it off after work. So how did the stone get in there?

Was it just passing, looking for somewhere warm to spend the night when it came across my shoe? Or maybe it materialised in there from some alternative dimension. Perhaps it's some cockroach's idea of a practical joke. Who knows?

Any ideas?