Science Buddies: "Ask an Expert"

I'm helping my little brother with his fifth grade science project, and we've already conducted the experiment. Our experiment showed that the more dimples in the Mentos, the shorter the geyser...But we can't understand why, because our hypothesis was opposite, we thought that adding dimples would increase the height of the geyser because we had created larger dimples for the carbon dioxide gas to fill and escape from. Can someone please explain to me why adding the dimples decreased the height of the geyser?

The key to what makes the reaction work so well is believed to be the increased surface area caused by the rough, nearly microscopic holes.

You didn't specify how you added dimples, or how large they were, but it might be that you actually reduced the surface area by adding dimples (therefore crushing some of the very small holes). Keep in mind much of the surface area you're working with is something you can't see with the naked eye.