The perfect rock!
Penguins don't live in the best habitats for finding nesting material, so they have to make do with what they can find.
Rockhopper penguins build their nests on steep rocky areas. To get there, they hold both feet together and bounce from ledge to ledge (imagine Winnie the Pooh's Tigger with wings and you've got the idea). These birds can bounce up to 5 feet!
Magellanic penguins dig burrows under the ground to form huge "cities" similar to gophers.
Adelies and chinstrap penguins use rocks to build their nests. The perfect rock is a rare commodity for these birds. They'll often fight over or steal each other's stones!