Living things refer to those organisms (plants and animals) which possess life and have the ability to eat, grow, respire, reproduce, obtain and use energy. They live in an environment where they can feed, grow, reproduce and obtain energy. Living things are able to sustain themselves from the environment they live.

One of the characteristics of living things is their ability to grow. This is an increase in size which is usually accompanied by development. For growth to occur, the cells need plenty of food that will provide energy and materials for building up new protoplasm. Growth in a living organism passes through some stages.

A chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation method, i.e., without breaking chemical bonds.Chemical substances are often called pure to set them apart from mixtures, i.e., a chemical substance can either be a pure chemical element or a pure chemical compound. A pure substance can be defined as a form of matter that has both definite composition and distinct properties.A common example of chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride) and refined sugar. However, simple or seemingly pure substances found in nature can in fact be mixtures of chemical substances. For example, tap water may contain small amounts of dissolved sodium chloride and compounds containing iron, calcium and many other chemical substances.

Non-living things are inanimate matter that do not possess life and therefore, cannot carry out the characteristics of living things. Non-living things undergo certain changes which could be temporary or permanent. Temporary change is a physical change while permanent change is a chemical change.

The word work is commonly used. Closely related to work are energy and power. In this chapter we shall study these concepts. All living beings need food. Living beings have to perform several basic activities to survive. We call such activities life processes. The energy for these processes comes from food. We need energy for other activities like playing, singing, reading, writing, thinking, jumping, cycling and running. Activities that are strenuous require more energy.Animals too get engaged in activities. For example, they may jump and run. They have to fight, move away from enemies, find food or find a safe place to live. Also, we engage some animals to lift weights, carry loads, pull carts or plough fields. All such activities require energy.Think of machines. List the machines that you have come across. What do they need for their working? Why do some engines require fuel like petrol and diesel? Why do living beings and machines need energy?