Brush-tailed penguins (the Adelie, Chinstrap and Gentoo penguin) love rocks. They collect rocks, they steal rocks, they Obsess ... over rocks. Because for these penguins, rocks are safety and security. Rocks are the currency of these penguin colonies. (I won't get into the varying stories of what may or may not be exchanged for rocks .... wouldn't want to pass on hearsay). Rocks are the building blocks of the penguins' nests. They help keep the precious eggs safe and dry as the snowmelt streams trickle down the slopes of penguin colonies in the Antarctic summer.
Here is an Adelie Penguin with a rock. It's got a small pile of rocks collected - it's building a nest. Never mind those molting penguin chicks in the background, which should serve as reminders to this chap that the summer season is coming to an end, and it's no time to be trying to gather rocks and start a nest. Still, many (inexperienced?) penguins are still collecting rocks, stealing them from neighboring nests, lovingly hauling them from whatever source they've found for themselves.