How do I make a photo "snap" on a grid line in photoshop?

Question: How do I make a photo "snap" on a grid line in photoshop!?

This is really hard to explain, but I will do the best I can!.

When I use the grid lines in photoshop, the marquee tool (and others) usually "snap" against the grid lines so the tool is the right size or so I can draw a perfectly straight line!. Now, it is not letting me do that!. Is there any setting I can change to fix this!?

Here is a picture that kind of explains what I'm talking about:
http://www!.flickr!.com/photos/22131398@N0!.!.!.

I am using Photoshop CS!.Www@QuestionHome@Com

Best Answer - Chosen by Asker:

there is a setting for snap to grid under the view heading
or
to draw a straight line!.!.i hold the option key down (but im on a mac)Www@QuestionHome@Com

To draw a perfectly straight line just hold the shift key while drawing!.Www@QuestionHome@Com