Carl Zimmer in Forbes.com:
For centuries, naturalists have observed animals doing something that looks an awful lot like playing. Horses kick and dance. Dogs wrestle each other to the ground. Warblers toss rocks. And when scientists give octopuses Lego blocks, they seem to have a lot of fun batting them around with their tentacles. But it is a supreme challenge to determine if they are actually playing in the same way Charlotte does, or if we are just projecting our own experiences on an alien animal kingdom.