Street Signs

John has been reading street signs for me on our morning drive to school.  He  reads the speed limit signs or stop signs.  This morning he saw a sign and said "That one says watch out for ones crossing the street."  I didn't understand him.  I responded by asking "Watch out for worms crossing the street? Really John?  Worms?"  "No Mom!  I said 'ones.'" Just then we approached a people crossing sign like the picture below. 

I asked John, "Is that the sign you saw?"  He said "Yes.  I said 'ones' because it could be other things besides people crossing the street."  I suggested "Oh, like dogs?"  John said "Yes, like people walking dogs.  Or wild dogs.  But if you saw a wild dog crossing the street, then you would see a dog catcher right on his tail."