**Question: **Tic-Tac-Toe is being played. One 'X' has been placed in one of the corners. No 'O' has been placed yet. Where does the player that is playing 'O' has to put his first 'O' so that 'X' doesn't win? Assume that both players are very intelligent. Explain

**Answer:**

"O" should be placed in the center.

Let's number the positions as:

1 | 2 | 3

---------

4 | 5 | 6

---------

7 | 8 | 9

It is given that "X" is placed in one of the corner position. Let's assume that its at position 1.

Now, let's take each position one by one.

* If "O" is placed in position 2, "X" can always win by choosing position 4, 5 or 7.

* If "O" is placed in position 3, "X" can always win by choosing position 4, 7 or 9.

* If "O" is placed in position 4, "X" can always win by choosing position 2, 3 or 5.

* If "O" is placed in position 6, "X" can always win by choosing position 3, 5 or 7.

* If "O" is placed in position 7, "X" can always win by choosing position 2, 3 or 9.

* If "O" is placed in position 8, "X" can always win by choosing position 3, 5 or 7.

* If "O" is placed in position 9, "X" can always win by choosing position 3, or 7.

If "O" is placed in position 5 i.e. center position, "X" can't win unless "O" does something foolish

Hence, "O" should be placed in the center.