Wet paint

I can't claim to have invented a perpetual motion machine (yet!) but I have come up with the perfect perpetual keep-a-four-year-old-entertained activity:

Give BallFiend an old paint brush and a bucket of water and tell him to paint the fence.

It is amusing to hear him call out, puzzled: "Where does it go? It keeps going away! Oh (annoyed), now I will have to start all over again!"
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