Place twenty-five matches in a row on the table. Request some one to
select one end of the row and to take one, two, or three matches from
it, you having the same privilege at the other end; and you guarantee he
will be compelled to take the last match no matter how he may vary the
number he takes.
The secret is to remove four matches each time between you. For
instance, if your opponent takes three you take one; if he takes two you
take two; if he takes one you take three and so on. It is obvious if
four matches are taken six times one match will be left on the table,
which your opponent must take.