that every crook can make or that he can buy in certain places that
every crook knows. Now, here is a check stamped in perforated
characters $300$. I take my little punch and fit into it a cutter that
will punch holes of the same size as the holes in the perforations.

"Now I punch out of the edge of the check a few tiny disks. I moisten
the tip of a needle and press them carefully into the holes that make
the upper part of the figure 3. See, even in my haste and without
glue, they fill the perforations completely and I can shake and pull
the check without disturbing them."

It was true. The little plugs fitted perfectly, and even with the
knowledge that they were there it was almost impossible to see where
they had been inserted.

"Now," continued the expert, "I merely take my punch and carefully
punch enough holes to the right of the upper part of the figure 3 to
make it a 5. And there you are. If I wanted to pass this check through
the bank I would only have to complete the job by smearing a drop of
the invisible glue over the back where I have plugged the original
holes. This glue is wonderfully tenacious and will actually hold the
edges of paper together. It needs only the smallest surface in order
to get hold. After it is on not even the microscope could detect it
readily. And no amount of pulling or shaking of the check will disturb
it.

"You may suppose that a check that is stamped this way, for
instance--$600$--would be hard to change into one of four figures. But
it is almost equally easy. The crook simply punches out enough disks
from the edge to fill up the last dollar mark completely, and after he