THE BOAT THAT FLOATS

First, fill a sink or washtub with water and ask your child to drop a marble into it. Then ask her to do the same with a lump of modeling clay. Once she's watched these objects sink, tell her that both the marble and the clay are made of materials that are much more compact, or denser, than water.

Next, ask her to mold a small rowboat out of the same lump of clay. Show her how to build up the sides of the vessel and have her hollow out the inside. Once everything's shipshape, have her try to float the craft.

She may be surprised to see that the same clay that sank just minutes ago will now float. Explain that the craft is buoyant because it is filled with air--the combined densities of the clay and air are less than the density of the water.

To see how buoyant her boat is, your junior shipbuilder can add a cargo of marbles one at a time until the craft sinks. Then, she can rework the clay into a few different-shaped boats to see which model will carry the most marbles.

THE UNSINKABLE EGG

Here's a neat trick that will show your child why it's easier for an object to float in the ocean than it is in a fresh-water pond.

First, fill a large glass to within a couple inches of the rim with hot tap water. Using a spoon, ease a fresh egg into the water. It will come to rest at the bottom of the glass because it has a greater density than water. Next, stir some table salt into the water until the egg rises. The salt increases the water's density, making it heavier, volume for volume, than the egg. As a result, the egg now floats.