179121479125786First_Children__s_Encyclopedia

239 Light beams Unless it enters your eyes, light is invisible. The beam of light from a lighthouse can only be seen from the si ...

240 Every sound starts with a vibration, like the quivering of a guitar string. The vibration squeezes and stretches the air bet ...

241 Sound No – dogs can hear higher notes than people, and squid can’t hear at all. Speeding sound All sounds travel at the same ...

It can be difficult to make an object move, but once it is moving, it will go on moving until something stops it. Force is neede ...

Forces and motion Become an expert 234-235 What is energy? 244-245 Machines Accelerating is fun, but in science it can be confus ...

244 Wheel and axle An axle goes through the centre of a wheel. Together they work as a simple rotating machine that makes it eas ...

245 Machines Lever, wheel and axle, gear, wedge, inclined plane, and pulley. Pulley A pulley makes it easier to lift something s ...

Which is the biggest ocean on the Earth? Planet Earth 246 Earth as a magnet Have you ever used a compass to find your way? It wo ...

Our planet The Pacific Ocean. It covers nearly half of the Earth’s surface.^247 What’s it made of? The Earth is made up of an ou ...

248 Earth is the only planet in the Solar System that can support life, because it’s just the right distance from the Sun. Our a ...

249 Making mountains The Himalayas started to form 50 million years ago, when two moving plates collided. The mountains are stil ...

250 The Earth’s crust is made up of different rocks. Some of these are hard but others are soft and crumbly. They are formed in ...

251 Rock and minerals Mineral mixtures Granite rock is made up of different coloured minerals. The black mineral is mica, the pi ...

252 The surface of our planet never stops changing. Over millions of years, land is slowly worn away by wind, rain, and rivers. ...

253 Worn by wind Strong winds can lift sand off the ground and blast it hard against rocks. The rock is worn into strange shapes ...

254 Soil is the thin layer of loose material on the land. Soil contains minerals, air, water, and decaying organic matter. Soil ...

255 Soil erosion When soil is farmed too much, its nutrients get used up. The topsoil blows or washes away. Not many plants can ...

The ground holds many useful things, from fuels like coal and oil, to drinking water, and building materials. These valuable ite ...

Creating concrete Concrete is an important building material. It is a mixture of sand, gravel, cement, and water. All these ingr ...

258 Earth is often called the blue planet because 75 per cent of its surface is covered in water. Most of the Earth’s water is s ...