Sandra Hibbs, age 12, of Peoria, Illinois, for her question:

 Why dogs a pebble sink and a big boat floats?

Big ship floats and a small pebble sinks. Yet the ship of iron and steel weighs far more than the little drowned pebble. This mystery makes sense when we compare the weight of the pebble with an equal amount of water, the weight of the ship with an equal volume of water.

The weight of a cork is, say, one quarter the weight of an equal volume of water. One quarter of the cork is in water, three quarters floats above the water. The water displaced by the floating cork is equal in weight to the whole cork. Ice is nine‑tenths the weight of water. So nine‑tenths of an iceberg is under water. This volume of displaced water equals the weight of the entire iceberg.

A cork is light for its size because it is riddled with air pockets. A big ship, too, is filled with chambers and pockets of air. It is light for its size, lighter than the same volume of water. A 1,000 ton ship is one that displaces 1000 tons of water. It floats because it occupies more space than 1,000 tons of water. The solid little pebble sinks because it occupies less space than an equal weight of water.