Today my 16 month old DS bit my 5yo DD, leaving a bruise and teeth marks. It's the second time he's done it. He's also bitten me in the past, but not as hard. I responded with a stern 'NO!" which stopped him in his tracks, but I'm not sure how I should handle this if it happens again. He does understand some things I say to him (e.g. if I say 'naughty' in a stern voice I'm sure he understands, he doesn't like it at all). Also can you think of guidance I should give my DD on what to do with him if she gets bitten again - this time I didn't immediately notice him do it, she was just quietly squirming in pain, with his teeth clamped to her leg! On the whole she's very tolerant of him, but she shouldn't have to tolerate this!