Triomino problem

We have a chess board (64 squares) and 21 triominos (63 squares, the triominos has the "I" shape). It is possible to cover the chess board with all the triominos, meaning only one square will be uncovered. Prove that this square is always one of the red squares:

EDIT: Just to make things clear, these are triominos:

http://upload.wikimedia.org/wikipedia/commons/2/22/Trominoes.svg

The shape we are using is the left one.

Pls tell the proof of this quest.

To prove this we colour the board in the following manner:

We can see that one triomino will always cover one, and only one blue square.

But since we have 22 blue squares, and 21 triominos, the uncovered square must always be on one of the blue squares.

Now we colour it again, same as before but mirrored:

Same applies here, we have 22 yellow squares, the uncovered square must be a yellow one.

If we combine them we get this:

Since it must be true that:

• The uncovered square is yellow
• The uncovered square is blue

The uncovered square can only be one which is both yellow and blue, which are those exact points.