If I have two candy bars that are the same size, I am sure the kids can pick which one will float and which one will sink. And they will probably be right. But what happens when I use a really big 3 Musketeers bar, and a really small Snickers bar? I’ll try it tomorrow and let you know. But, I will be very surprised if they understand that with density, size does not matter. Only “crowded-ness” matters.