Libby and I did the Rainbow Density kitchen experiment over the weekend (instructions here). It was fun to begin to talk with her about science and how experiments are performed (we were careful to measure and observe everything that was happening).
I am not sure Libby really understood my attempts at explaining density; understandable given my disjointed first attempt. But I think it made more sense when I said the sugar molecules were "hugging" the water molecules, and as we add more sugar the water becomes "heavier" (since more sugar is hugging it). I'm greatly looking forward to more experiments as Libby (and Cooper) grow up.