9 Tunnels are round

What are houses made from? Most houses are made from bricks, but they can be built out of almost anything—as long as it's strong and it keeps out the weather. Builders like to use materials they can get hold of easily bricks or stone, wood, reeds or mud. Bricklayers put mortar between the bricks. Mortar is a mixture of water, sand and cement, and it glues the bricks together. Houses protect us from the heat in the summer, and the cold and rain in the winter. There are about 12,000 bricks in the outside walls of a two-storey house. Some birds use mud to build their homes, but it's not shaped into little bricks! There aren't any bricks in the middle of a forest, but there is plenty of wood for building cabins. For thousands of years in the marshes of southern Iraq, the East, people used bundles of river reeds to build beautiful homes. In hot places such as Africa, people often built with mud and straw, which dries hard in the sunshine.

What holds up the ceiling? The weight of ceilings and floors is carried by strong wooden beams called joists. There are joists hidden away under the floorboards and above the ceiling of every room. What's behind the walls? Among other things, there's usually a lot of air! That's because most brick houses have double outer walls, with a gap between the walls. Pipes and electrical wiring are tucked away in this gap. What's under the floor? There are pipes under the floor. Some carry clean water around the house. Others take dirty water away. Some aren't for water at all-they carry gas, which is burned to heat ovens for cooking and water for washing. The roof is held up by thick wooden tufters. Tiles keep out the rain. Sometimes uninvited visitors live under the floor, such as the wood-eating grubs of the woodworm beetle. Building a house is teamwork. A plumber lays all the pipes, an electrician does the wiring, and a carpenter puts in the windows, doors and cupboards.

What do bulldozers do? Bulldozers clear building sites and make the land flat enough to build on. The huge steel blade at the front shifts great mounds of soil, brick or anything else that's in the way. Bulldozers are incredibly powerful machines—just the job for pushing and shoving things around. Why don't bulldozers get stuck? Instead of a tyre on each wheel, a bulldozer has a crawler track around each pair of front and back wheels. Crawler tracks grip the ground well and stop the bulldozer from getting bogged down in the mud. At the back of some bulldozers there's an arm with large metal spikes called rippers. These are used to break up hard rock and to tear out tree stumps. Guess what a small bulldozer is called- a calfdozer! There are even special under- water bulldozers which flatten riverbeds or the seafloor so that bridges can be built on them. A bulldozer once won a tug-of-war contest against 150 people! Bulldozers can be as much as 500 times heavier than you. But their tracks spread the weight so evenly that they are much less likely to sink in the mud!

Why do houses need holes? Most houses start with holes in the ground. The holes are filled with runny concrete, which hardens as it sets. The rest of the house is then built on top of this firm base, called the foundations. What do diggers do? A digger does much more than dig. Its wide front bucket is brilliant at scooping rubble into a truck. A scissor-like bucket can also be fitted to the front, to grab and lift logs and pipes. Many diggers have a wide front bucket, as well as the digging bucket at the back. A digger can dig a trench as fast as 20 people with spades. The biggest earth-scooping buckets can hold five large family cars. They are on huge mining machines called excavators, which are about ten times bigger than the average digger. Diggers also dig holes for swimming pools, rubbish heaps and top-secret underground hiding places! A huge hammer can be fitted to a digger, allowing it to smash up rocks.

Do buildings have roots? Skyscraper foundations are called piles and they are made of steel or concrete. Piles do the same job for a skyscraper as roots do for a tree. They stop it blowing over in the wind, or sinking into the ground under its own weight. Which are the world's biggest hammers? To knock a nail into wood, you use a hammer. To drive a steel pile into the ground, builders use a huge hammer called a piledriver, A crane lifts up the piledriver, then drops it on to the pile with a deafening BANG! Which are the world's largest corkscrews? Sometimes, the holes for piles are drilled with an auger—a long tool which works like a giant corkscrew, twisting its way deep down into the ground. A steel rod is put into the hole, and then lots of concrete is poured down around it. The piles for some skyscrapers are well over 50 metres deep. That's the same as five houses buried one on top of the other! Using piledrivers at night is against the law in many towns and cities. They're much too noisy.

Do buildings have a skeleton? Nowadays, most big buildings have a strong steel framework inside them. This framework does a similar job to the bony skeleton inside your body. It props up the building and stops it from collapsing to the ground. Do buildings have a skin? The skin of a building is formed by its windows and walls. On modern skyscrapers these are made of glass or plastic panels, and thin concrete or stone sheets. These panels and sheets are called cladding. by big ncils called rivets. These are punched through holcs. Then their ends are flattened to stop them slipping out again. Other beams are joined by welding. The ends are heated until they melt and flow together like runny chocolate. When they cool down, the joined ends have hardened together. A skyscraper's framework is made from steel beams and tubes. The ends of one kind of beam look like the letters I or H. Beams this shape are stronger and lighter than square beams.