I see. So maybe it'd go a little like-- [she uses the tip of the shovel to draw a circle around the outside of the sandcastle,] --this? We'd dig it out and then fill it with water, right?

[A pause. Eventually, the water would all seep out, as the sand absorbs it...is there a way to keep that from happening? She taps her chin thoughtfully and then--] ...Oh! We could line the bottom with shells, and that might slow down the sand's absorption rate.