Things that sink: anchors, pirate treasure, bowling balls, water-logged cardboard, containers with holes. Things that don’t sink: tin foil flattened out and crimped on the edges, balloons filled with air, ships built such that their weight can displace an equivalent amount of water. Archimedes explained this better. Sinking versus floating involves density, weight dispersion, gravity, materials and experiment.
The Enrichment Class at Orange Avenue Elementary School in Milford built ships recently with grand hopes that they too would join the elite list of unsinkable things.