THE MOUSE-TRAP PUZZLE

This is a modern version, with a difference, of an old puzzle of the same name. Number twenty-one cards, `1, 2, 3`, etc., up to `21`, and place them in a circle in the particular order shown in the illustration. These cards represent mice. You start from any card, calling that card "one," and count, "one, two, three," etc., in a clockwise direction, and when your count agrees with the number on the card, you have made a "catch," and you remove the card. Then start at the next card, calling that "one," and try again to make another "catch." And so on. Supposing you start at `18`, calling that card "one," your first "catch" will be `19`. Remove `19` and your next "catch" is `10`. Remove `10` and your next "catch" is `1`. Remove the `1`, and if you count up to `21` (you must never go beyond), you cannot make another "catch." Now, the ideal is to "catch" all the twenty-one mice, but this is not here possible, and if it were it would merely require twenty-one different trials, at the most, to succeed. But the reader may make any two cards change places before he begins. Thus, you can change the `6` with the `2`, or the `7` with the `11`, or any other pair. This can be done in several ways so as to enable you to "catch" all the twenty-one mice, if you then start at the right place. You may never pass over a "catch"; you must always remove the card and start afresh.