Actually, that sounds really, really boring. But if you instead tell them you are going to cause a giant geyser of soda to erupt in the driveway, they will get pretty excited. Then they will ask what happens if you use different sodas, and then different candies, and suddenly a little experimental physicist or chemist is born.
I'm talking about the Coke - Mentos experiment which has been a staple of science classes since...no one knows when. As is often the case with things like this, there is no record of the first time it was done. We do know it used Wintergreen Lifesavers originally, until the candy company made them too big to fit in the bottle - imagine that, a company making a candy bigger for the same price - and then Mentos was adopted instead.
To do it in its simplest form, you just need carbonated soda and some Mentos. Why does it happen? Some of it is chemistry but some of it is also plain old physics. Instead of looking at a Mentos and seeing something smooth, if you look under a microscope you will see it has dimples or pores. Having that extra surface area is key. A smooth ball has just the surface area of the ball and a smooth ball actually doesn't travel all that well for a number of reasons. That is why golf balls have those dimples on them.
For chemistry, something with a lot of pock marks or dimples or pores has more surfaces and those are ideal nucleation sites - places for gas to form and create bubbles. That's what happens when the CO2 gets going. And Mentos candies are heavy so as they are sinking to the bottom, the CO2 is reacting with the candy and creating even more nucleation sites on it.
First, here is our version. Obviously there are some things you may not anticipate and we did not anticipate every variable, like an elementary school boy. In testing everything went perfect and so we used a second bottle for the video take and:
Why does Mentos work so well? In addition to the CO2 and the nucleation sites causing a lot more bubbles to form, the gum arabic in it is a surfactant, it lowers the surface tension in the Diet Coke. What about other types of soda? If the Mentos is the same, and the CO2 in other soda is the same, they will all be the same, right? Sort of - some carbonated drinks are more equal than others.
Diet Coke does seem to work better.
Mentos-soda experiment using 5 Mentos. From left: Perrier, Coke, Sprite and Diet Coke. The green marks are with 0.5 m separation. Credit: K. Shimada on Wikipedia
Why? We mentioned the surfactant in Mentos but Diet Coke has that also - aspartame reduces the surface tension of the liquid more than the sugar or corn syrup in Mexican or American Coca-Cola will. If you don't intend to drink the soda at any point, you can add dishwasher soap for even greater surfactant power.
The other factor in making a geyser that really goes up is by wisely taking advantage of the pressure that bubbling CO2 creates. If you just drop the Mentos into the bottle it will gush out all over the place but if you want it to really skyrocket, you basically make it a...rocket. By making a small hole in the bottle cap, you will force the bubbles to escape through a much smaller area and that pressure causes it to go crazy.
Obviously if you do that, you need to put the lid on really fast and that isn't always easy. So I used Steve Spangler's Geyser Tube, which is a very pleasant $5. Most importantly, you screw it on the bottle the kids load in the Mentos, and then they pull out the pin and it drops the Mentos in for you. Because the narrow opening tube is already in place, you can just have fun.
The more Mentos you use, the less obvious the change in the Mentos will be. If you use two, for example, a kid can see all of the pock marks very clearly. 5 will get you a better geyser but the marking on the Mentos won't be as obvious.
The Mentos will taste the same afterward and the kids can drink the leftover Coke but it will be flat, which makes the point about the CO2 action. It will also taste a little Mentos-y.
What was the fallout from the science experiment? A few days later there was a party at a neighbor's house and the mom had rented a Mythbusters DVD that had their version of the experiment. Sure, the kids watched "Frozen" and whatever at the party too, but these elementary school kids were also watching Mythbusters and telling the guys on TV how to get the best gusher.
So what ever happened to Wintergreen Lifesavers once they got replaced by Mentos? Not to worry, they are not out of it yet. Next time, we are going to use them to create lightning in our mouths.
Tonya Shea Coffey, Diet Coke and Mentos: What is really behind this physical reaction? Am. J. Phys. 76, 551 (2008); DOI:10.1119/1.2888546
Diet Coke and Mentos: What is really behind this physical reaction? presentation by Tonya Coffey
Why Do Diet Coke and Mentos React? By Daven Hiskey MentalFloss.com