Now, when I went to explain it, I told him that the bubbles basically lifted the raisins up and when they popped at the surface, the raisin sank again. I had some vague notions of density and water displacement, but reading the science about it in VanCleave's book, it's actually a pretty simple notion: buoyancy. The bubbles make the raisins buoyant and thus bring them up to the surface — VanCleave compares this to deep-diving ocean vessels that rise and dive by changing their buoyancy. She kept the experiment simple, with just one glass with the seltzer as opposed to the tap water and just 5 raisins, which made it easier to keep track of each raisins. Since I wasn't going for hard science with Conor, and was also looking to buy time, I just let him throw in raisins into whatever glass he wanted. The upshot of that was that he had two containers to play with, more raisins to make a mess with, but ultimately, he had a comparison. The raisins in the flat water went down to a murky glass bottom with no glorious bubbly dance to accompany them.
Eventually, I'd like to get to experiments on water displacement and explore how big ships don't sink even though they're made of metal and also get into phase changes as Conor has been recently interested in how ice is water, but it's frozen(!). In the meantime, this was a good experiment and fun (and useful) for both of us.