Periwinkles are the pebbles of the sea. You find them along the entire coast, on breakwaters, dikes, piers, etc. When a periwinkle ventures high onto the dike on a hot summer day, it lets itself roll down the slope when it gets too hot, hoping to land in water. As it crawls over a hard surface, it leaves behind a trail of slime. All kinds of algae stick to this slime. Other snails, such as laver spire shells, graze upon these slimy trails. If you pick up a periwinkle, the snail will hide in its shell. Pick up several snails and shake them in your hands, then the snails often crawl out of their shell. They confuse the shaking with wave movement. They think that the tide has risen and it is time to look for food.