Cats will often eat grass and leaves as part of their natural diet as apparently it helps them with digestion and the regurgitation of fur balls. That's why indoor cats should have at least a tray or pot of grass (the green type, not the other stuff!!) that they can graze on. My naughty fella will rip the leaves of my pot plants if he's been told off or I'm not taking any notice of him, just like a naughty child.