The biting stage!

I'm a nanny, not a parent, but I see the upset my son and his wife are going through at the moment as my grandson, 17 mos, is biting, both them and other children. They are trying to handle it as best they can (they have a naughty chair). To me he is still a baby and probably doesn't understand fully why it's naughty. Anyone have any bright ideas?

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