Have you ever had ice? How does it feel, cold isn't it? Ice comes from chilled water. For example, if water whose temperature is 20°C is cooled, its temperature will drop to 19°C, 18°C, 17°C, 16°C, and so on. When the temperature is 0°C, water will freeze into ice. If ice is cooled further, its temperature will drop again, for example, 1°C below zero, 2°C below zero, 3°C below zero, and so on.

How to write numbers under zero? Numbers below zero are written with a negative symbol (–).

1°C below zero is written –1°C

2°C below zero is written –2°C

3°C below zero is written –3°C

Numbers consisting of negative numbers, zero, and positive numbers are called integers. So, integers are: ..., –4, –3, –2, –1, 0, 1, 2, 3, 4, ….

In everyday life there are many things related to integers. For example, the height of a place 2,500 m above sea level. All places on earth, the height is measured from sea level. So:

• Sea level is 0 m high.

• Place above sea level is positive (+).

• Place below sea level is negative (–).