how to insert "|" in org-mode table

I have some trouble in using org-mode. I want create a table in org-mode like this:

| operator | use         |
|----------+-------------|
|   |      | expr | expr |

but when I press TAB, the table changed like this:

| operator | use |      |      |
|----------+-----+------+------|
|          |     | expr | expr |.

How to avoid it?