Dennis Caruthers, age 14, of Watertown, N.Y., for his question:

WHEN WAS THE FIRST HOUSE BUILT?

We don't know when humans built the first house, but we know it was in prehistoric days, before records were kept.

We are quite sure that humans' first shelter was in a tree. People learned that tall trees offered protection from animals and also kept off some of the rain. Later they learned to pile branches to make crude shelters from the wind. And still later, some humans lived in caves.

Finally, they learned to make simple tools. At first these were made of stone. Later they were made of metal. The tools helped people build better shelters.

Where there were few trees, humans learned to pile up stones to make shelters. And they also learned to form clay into small blocks that could be dried in the sun. Shelters were then built by piling up these sun dried blocks on each other.

Before 4000 B.C., lake dwelling people learned to make their houses over water. They drove heavy logs into the lake bottom near the shore and built platforms atop the logs.

Ancient Egyptians started building flat topped homes of sun dried brick around 3000 B.C.

The Assyrians also adopted the sun dried brick method of building about 2500 B.C. But the Assyrians discovered that putting bricks into fire made them harder and stronger.

The ancient Greeks made stone houses with slanted roofs. They were the first to put doors in their buildings.

The Romans copied many Greek ideas, but they were the first to install central heating. This was accomplished by putting rows of earthenware pipes under the floor and running hot air or hot water through them to heat the floors and rooms.

The Romans also added small windows to their walls, and they were the first to use glass windowpanes.

German and Scandinavian tribes later introduced buildings with frameworks made of heavy wooden timbers.

Early German and Scandinavian tribes built roofs with high peaks, which they covered with layers of bark and then layers of sod. They also dug ditches and built earthen walls around their homes to protect them from animals.

In the Middle Ages, castles were built with thick stone walls, water filled moats and drawbridges.

Around the mid 1300s, European nobles became the first to build fireplaces in their castles.

About this same time, Europeans began to build half timbered houses with brick or stone foundations. Tree trunks were placed at each corner of a house, with beams placed between the trunks. Walls were then covered with lath and plastered with a mixture of clay and straw.

By the 1500s, people were building half timbered houses that were three or four stories high.