Another way to do it, if you are working in 2007, is set up the text animation to be bold and the duration in the animation pane, select "Until next click". Then have a second animation (of a small circle or something) that starts "On Click", I used Appear entrance animation. Move the circle off of the slide to the left so that when you click, your text goes back to normal and the circle appears off the slide so noone can see it. I tried to make the circle invisible (no fill and no outline) but that didn't work.

Hope that helps,
Math Nerd