"Life" can be thought of in purely chemical terms as a complex set of chemical reactions occurring in the cells of the organism. Cells are like small factories all doing their jobs so that the overall corporation, the body, is an efficient organisation. The total of all these reactions creates a moving, respiring, reproducing organism which can respond to changes in its environment, helping it to survive. Some organisms carry out these functions in a single cell, but most are multicellular.

What do living things need to ensure the chemical reactions keep going?

Food
Food is needed firstly for energy to help the chemical reactions to occur. This then allows muscles to work for movement (including the heart beating); "building" processes for repair and growth; and messages to be sent in the nervous system.

Food is also needed as the source of the basic chemicals (nutrients) required to make new cells. Plants can make their own food by photosynthesis using sunlight, carbon dioxide and water. They then use this, along with soil nutrients, for their own energy needs.

Oxygen
Oxygen is needed to combine with simple sugars in a process called cellular respiration. This occurs in the cells and provides energy. Oxygen is essential for plants and animals alike, though some bacteria can survive without it. A drowning person dies from lack of oxygen. No oxygen... no energy... no life. Fortunately, plants produce oxygen as a by-product of photosynthesis, which means that, as long as we have plants, we won't run out of oxygen.

Water
Our bodies are about 80% water. Most chemical reactions must occur with the chemicals dissolved in water. Water also helps transport substances inside the body, remove wastes, and aids in fertilisation of the eggs by sperm. In plants, water is essential for photosynthesis. In extreme hot and dry climates, humans can dehydrate and die within a few hours.

Warmth (correct temperature range)
The chemical reactions needed for life work best at a particular temperature. In many animals, this is around 37°C. Lower than this, the reactions work too slowly to sustain life. If it's too hot, the enzymes helping speed up the reactions are denatured (basically "cooked"!) and again the system is too slow.

Plants can survive at lower temperatures with a slow metabolic rate as they don't need to move about for food or mating.

"Warm-blooded animals" (scientists call them homoiothermic or endothermic) such as birds and mammals have an internal source of body heat, and feathers, fur or hair to assist in keeping their body temperature constant all year round so that they are always active and survive well even in cold places.

"Cold-blooded" animals (poikilothermic or ectothermic) cannot keep their body temperature constant (it goes up or down with the surrounding air temperature) which means they are slower or inactive in colder conditions until they can warm up (e.g. butterflies spread their wings to the Sun in the morning, and lizards lie on the warm roads).

Protection
Shelter is needed to maintain warmth, to prevent drying out, and for protection against predators.