I like hydrogen. And I thought it might be interesting to separate some out from water, with the hope that I could fill two balloons (one with oxygen, one with hydrogen), and later ignite them to see which gave the more interesting burn (the potential boom). After some failed setups, I decided against separate collection; besides, putting the H2 and O2 in the same container would provide a ready source of oxygen for the combustion.
The kids were reading a book which mentioned photosynthesis, and they asked how it works. I gave them the quick “the plant turns sunlight, water, CO2 into sugar, which it uses for engery.” That wasn’t enough for them. SO I went to the chalkboard and did a quick stoichiometry; then I got out some handy Duplo® bricks, made some water and CO2 molecules, which I put on a Duplo® base, to act as the leaf. Our hands became the sunlight, and we … disassociated … those molecules. 🙂 Then I rebuilt them into sugar* molecules that went deeper into the plant, and O2 molecules back into the air. They seemed to like that explanation.