Work distinguishes man from the rest of the creatures.

Work is one of the characteristics that distinguish man from the rest of the creatures, whose activity for sustaining their lives cannot be called work. Only man is capable of work, and only man works, at the same time by work occupying his existence on earth. Thus work bears a particular mark of man and of humanity, the mark of a person operating within a community of persons. And this mark decides its interior characteristics; in a sense it constitutes its very nature.

John Paul II, Encyclical Laborem Exercens

This had never occurred to me before. I've seen a lot of things singled out as distinguishing us from animals but not work.