Feeling warmth
Have you ever stood by a stream on a warm, sunny day and thought about paddling? When you decided to paddle you took off your shoes and socks and stood barefoot on the rocks by the water. The rocks felt warm but when you dipped a foot in the stream the water felt almost painfully cold.

The skin is the part of the body that detects cold just like the eye detects light and the ear detects sound. You may think that the skin also detects coldness but this is wrong. When we say that something is cold our skin has really told us that the object lacks heat. The way the skin compares the heat in an object works like this. The skin gives out heat all the time. When it touches an object that is giving out more heat, this extra heat moves into the skin and the skin tells us that the object feels warm. If the object is giving out a much larger amount of heat than the skin, this also passes into the skin and the skin tells us that the object feels hot. When the skin touches an object that is giving out less heat than the skin, heat from the skin flows into the object. This loss of heat makes the object feel cool. If the object is giving out much less heat than the skin a large amount of heat will leave the skin. This makes the object feel cold.

Heat always travels in one direction. It travels from a warmer object to a cooler one. Your feet felt warm by the stream because heat was flowing into them from the rocks. Your foot felt cold in the water because a large amount of heat was flowing out of your skin into the water.

Where does heat come from in a fire?
The heat comes from the substances that are burning in the fire. When something burns it breaks up into other substances. For example, when wood burns, smoke goes into the air and ash is left behind on the ground. As the wood breaks up into smoke and ash it releases the energy stored inside it. This energy escapes in two forms. They are light and heat.

How did the energy get into the wood?
There are many forms of energy and one form can change into another. The energy in wood is stored energy. Originally it came from sunlight. The Sun releases some energy as light. When it falls on the leaves of plants, they make food and store some of it in their bodies. When sunlight shines on trees, their leaves make food and some of it is stored in their wood.

Does the Sun burn like a fire?
People used to think so. Some people in the past thought that the Sun was a huge piece of coal in the sky. When anything burns it uses up a gas in the air called oxygen. The Sun is 150,000,000 kilometres out in space where there is no oxygen so it cannot burn like wood and coal on Earth. The Sun is a huge ball of gases. Over a million planets the size of the Earth could fit inside it. At the centre of the Sun the gas is squeezed so hard that it changes into another gas. When this happens, heat and energy are released. The gas that is squeezed is called hydrogen. The gas that is made is called helium. You may have already met helium and not known it. If you have ever had a balloon that floated upwards when you let go of it, that balloon contained helium. It floats because it is lighter than air. The helium in your balloon was made here on Earth and not in the Sun.

Is it the Sun's heat rays that burn the skin?
No. The Sun sends out other kinds of rays as well as heat and light. Ultra violet rays are one kind of these rays. We cannot see them but some animals, like bees, can. Ultraviolet rays cause changes in the skin. Most of these changes produce sunburn. Some changes can cause more serious damage and can even kill. In the past the Earth had its own way of keeping large amounts of ultraviolet light from reaching living things. It had a thick layer of a gas called ozone high in the sky and all around the planet. Today the layer is much thinner and there are large holes in it over the North and South Pole. This means that dangerous numbers of ultraviolet rays can reach the ground and people should protect their skin with a special cream when they are out on sunny days.

Why do people use their elbows to test the warmth of water?
They use elbows to test the bath water for babies. The skin on the elbow is particularly sensitive to heat so it is dipped in the baby's bath water to test it. The water that is chosen for the baby's bath is about as warm as the body - 37°C.

How does the skin tell you water is getting warmer when you run the hot tap?
If the tap has not been used for a while the first water to leave it will have less heat in it than the skin of the fingers. This means that heat will flow from your fingers to the water and the skin will tell you that the water is cold. As the water continues to flow it will have more heat in it. This will mean that less heat leaves your skin and it will tell you that the water is getting warmer. When water with a large amount of heat in it is flowing from the tap some of this heat will enter your skin and your skin will tell you that the water is hot. You must remove your fingers before the heat burns them.