Shells in the ocean

So today we made shell necklaces . They all got to pick a shell and we took them outside and rub then on the ground and it makes a hole. We came in and decorated them just like Hermit the crab does in the story A house For Hermit Crab By Eric Carle

We talked about shells rubbing against rocks sand and other items in the ocean . What salt water and lake water does to the shell .