My husband thinks this is an urban myth, but it’s totally true. You CAN make a geyser by dropping Mentos into a Diet Coke bottle. This picture is from our alumni camp. What you can’t appreciate are the chants of, “Men-tos! Men-tos!” from the kids watching.
The activity is simple enough. Put a 2 liter bottle of Diet Coke on a stable surface so it won’t fall over when the geyser shoots. Open it. Then, as fast as you can, drop in as many mint Mentos (not the kind with the candy coating) as you can. We found that 9 was about as many as we could do with our hands. The reaction is so fast that the tenth piece of candy flies up with the geyser.
This is a great opportunity to try things with the kids. Let them make suggestions and act on them if you can. How many Mentos is optimal? Will things other than Diet Coke work? (Yes, they will, but you don’t get a really high geyser, so come prepared with sparkling water, regular Coke, and other things you think the kids might ask for.) What about different kinds of Mentos? (This doesn’t work, by the way.) Why does this work, anyway? And of course, you should let the kids taste the Diet Coke afterwards. Is it any different in taste or amount of bubbles? Why?
Our school owns one of those geyser tubes and we like to use it because it lets you drop the Mentos in nearly simultaneously with the little pin plus the nozzle makes the geyser shoot straight up. Unfortunately, we couldn’t find it! The supply room is being reorganized and some things just aren’t findable right now. But as you can see, you don’t really need that tube. It’s just for added fun.