How do you suppose there is water in the ground? We know that water comes down in dew and in rain (and snow), that it flows along in rivers and streams, and that there is water in lakes, oceans, and any other kind of basin where the shape of the land will allow water to collect. We don't really know how it comes to be down in the ground. We really don't expect water to be in the ground unless the ground is wet on top where we can see it.

It would appear, then, that some of the water that comes to us in the form of rain and snow is absorbed by the earth, instead of running off in rivers or collecting in lakes.

Absorption of water can be shown by dipping one end of a slice of bread (or one side of a sugar cube) into a container of colored water.

Materials like bread and sugar cubes contain many empty spaces and are said to be "porous."

Now fill a glass jar three-quarters full of dry soil. Pack it down tightly, then quickly fill the rest of the glass with water. What happens?

Do you see the bubbles coming from the soil? What makes the bubbles? In our experiments with water, we saw that if air is below the surface of the water, it forms bubbles and the bubbles rise to the top.

Since soil is porous, and the "pore spaces" are filled with air, when water replaces the air, the bubbles of air come to the surface. Obviously, some soils are more porous than other soils, and some pore spaces are larger than others.

Rocks have pore spaces, too, and can absorb water similarly to soils.

We can check the absorption rate of rocks by weighing a piece of granite and/or marble, and a piece of sand-stone and/or limestone. Record their weights, then place the rocks in a jar of water. At the end of the day, weigh each rock again.

You will find that some rocks are more porous than others -- the more porous rocks have absorbed more water and have gained more weight. For fun, try to wipe the rocks dry.