adobe photoshop help, brush strokes continuous, please help, will award best answer.?

ok i am using abobe photoshop cs3, when i use the brush on photoshop, and i color a place in on the picture with the opacity set to 40%, then when i let go of the mouse and click on a different selection again it makes the brush stroke darker than what it was before, i forget how to change it so that it will stay a continuous stroke all through the picture, even when i let go of the mouse to do another brush stroke, thanks