(related to Problem: A Juvenile Puzzle)
As the conditions are generally understood, this puzzle is incapable of solution. This can be demonstrated quite easily. So we have to look for some catch or quibble in the statement of what we are asked to do. Now if you fold the paper and then push the point of your pencil down between the fold, you can with one stroke make the two lines $CD$ and $EF$ in our diagram. Then start at $A,$ and describe the line ending at $B.$ Finally put in the last line $GH,$ and the thing is done strictly within the conditions, since folding the paper is not actually forbidden. Of course, the lines are here left unjoined for the purpose of clearness.
In the rubbing out form of the puzzle, first, rub out $A$ to $B$ with a single finger in one stroke. Then rub out the line $GH$ with one finger. Finally, rub out the remaining two vertical lines with two fingers at once! That is the old trick.
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