Take a sheet of A4 paper:
Fold it in half:
Make cuts like this:
(DON'T cut right to the edges -look carefully at
the lines!)
Now cut along the fold (where the
red line is)
- but DON'T go right to the edges or you'll just
have a heap of scrap paper!
Now open the the paper out carefully and amaze yourself by climbing through the hole you have cut.
To be even more amazed, try it with a postcard - it's still possible to climb through the hole!