The first of the How Does That Work? series on the Science Matters blog. I will periodically show you products and demonstrations that you can present to your students and challenge them to explain the science of how they work. Make sure you decide ahead of time what you'll accept as a valid explanation - can it be printed straight off the internet, written in the student's own words, or does the student need to be able to explain it to you conversationally? What will a valid explanation earn the student - a prize, extra credit, a feeling of goodness?
I try to set out a new How Does That Work? or What is That? (more on that soon) challenge about once a month. It would be great to switch it up even more often, but I haven't yet built up my repertoire quite enough.
The Potato Clock! Set one up and amaze your students. Change the power source regularly - all sorts of fruits and vegetables will work to power the clock, as well as some liquids! Challenge your students to explain how the potato clock works.
Here is one explanation that might help you and your students understand the process. The internet is full of explanations - some geared to younger students and some to older. Find something that meets your needs.