LOWKEY SMART

When you round, look at the digit to the right of the one you are rounding to: 5 or more rounds up.

Scientists sort living things into groups by looking at their features. Two big groups are plants and animals. Animals with a backbone are called vertebrates; animals without a backbone are called invertebrates. There are five groups of vertebrates: fish, amphibians, reptiles, birds and mammals. Mammals have hair or fur and feed their babies milk. Birds have feathers and lay eggs. Insects are invertebrates with six legs, and spiders are invertebrates with eight legs. A classification key asks yes/no questions about features to help you name a living thing.

A habitat is the place where a plant or animal lives. Habitats can change, and the change can be good or bad for the living things there. Building new roads and houses can destroy habitats. Litter and pollution can harm animals. People can also help: nature reserves protect habitats, and planting trees gives birds and insects new homes. Some changes are natural, such as floods, droughts (long periods with no rain) and the changing seasons.

Materials can be solid, liquid or gas. Solids keep their shape. Liquids flow and take the shape of their container. Gases spread out to fill any space they are in. Heating can turn a solid into a liquid (melting) and a liquid into a gas (evaporating). Cooling turns a gas into a liquid (condensing) and a liquid into a solid (freezing). Water freezes at 0°C and boils at 100°C. Temperature is measured in degrees Celsius (°C) using a thermometer.

Sounds are made when something vibrates, which means it moves back and forth very quickly. The vibrations travel through a material, usually the air, to your ear. Sound can travel through solids, liquids and gases, but not through empty space, because there is no material there to vibrate. Pitch is how high or low a sound is: on a guitar, a shorter or tighter string makes a higher pitch. Volume is how loud or quiet a sound is: hitting a drum harder makes bigger vibrations and a louder sound. Sounds get fainter as you move further away from them.

Electricity flows around a circuit. A simple circuit needs a cell (often called a battery), wires and a component such as a bulb. The circuit must be a complete loop with no gaps for the bulb to light. A switch opens or closes a gap in the circuit: when the switch is open, the circuit is broken and the bulb goes off. Materials that let electricity pass through are called conductors; most metals, such as copper, are good conductors. Materials that do not let electricity pass through are called insulators, such as plastic, wood and rubber. Mains electricity from plug sockets is dangerous and must never be played with.

Digestion breaks food down so the body can use it. It starts in the mouth, where teeth break food into small pieces and saliva makes it wet. The food goes down the oesophagus (food pipe) to the stomach, where it is churned and mixed with acid. Next, the small intestine takes the useful nutrients into the blood. The large intestine takes water out of what is left. Humans have three types of teeth: incisors for cutting, canines for tearing, and molars for grinding and chewing.