Inside the prickly outer casing that fell from a horse chestnut is a conker. I picked this one from the ground beneath a tree today and brought it home. I ran a knife carefully along one of the plates that make up the case, to reveal what is within.
And here is the conker, with its outer casing split apart. As you can see, there is just a single conker. When I picked it from the ground I didn’t know whether it would have a single conker within or, like others that I found, would have three conkers occupying it.