I put several sea shells of uncertain provenance out on my deck several months ago. Their sole intended function was to enliven the otherwise drab wooden railing. However, certain arthropodous denizens of the backyard, distantly related to the hermit crabs of the sea and the seashore, appear to have developed a liking to the shells.
When I lifted one snail shell up yesterday, I inadvertently revealed the hideout of a spider of some sort.
I don't know if the spider was using the shell to hide from unsuspecting prey or its own predators or simply as a shelter from the elements. The correct answer may be all of the above.