Try this example.

Now if you fill in as many of the numbers and their two possibilities you
will get to this point and cannot find

any more. There is a way to continue even though it looks like you are
stuck. Try to figure out how to keep

going before looking at the last picture of the puzzle.

Solution: read below if you cannot find it.

In the lower right box the 5 and 9 have two squares locked up.
The remaining numbers that go in that box

are 1, 3, and 8. They go in the lowest three squares of the column
that goes down the middle of the

right hand boxes. The other numbers missing in that column is 2,
4, and 6. They go in the upper two boxes.

The 6 will go in the top square because 2 and 4 are in that row.
We would not be able to solve this

without knowing the 5 and 9 lock up the two squares. Here is the
finished puzzle solved.