Herding Cats

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There is a phrase " as hard as herding cats" meaning difficult.
To be honest herding them was not too difficult, getting them to stand still and smile at the same time was impossible, as you can see.
In the house we have a picture hanging on the wall that is similar to this one , except there is only three grandchildren. Number four arrived five years ago but I have not got round to updating the picture. 
I now know why.