Positive and negative integers are commonly used. Examples are:

**Bank account balances**- A positive bank account balance means that you have savings; A negative balance means you owe money.**Temperatures**- Water freezes at 0 degrees Centigrade and boils at 100 degrees Centigrade. Positive temperatures are warmer than ice. Negative temperatures are very, very chilly.**Height above sea level and depths below sea level**- The coastline is zero. Land height is positive (for example, Mount Everest is +8850 metres). Ocean depths are negative (for example, The Marianas Trench is –2550 metres).**Elevator floors**- The ground floor is zero. Going up to the penthouse is positive. Going down to the basement is negative.

**Q1.** The temperature of the propellant gas is an aerosol can is –25° Centigrade. When it was sprayed into the air on a day when the temperature was +20° Centigrade. By how many degrees did the spray increase?

**Q2.** Mount Everest is 8850 metres above sea level. The Marianas Trench is 2550 metres below sea level. What is the height difference between them?

**Q3.** I live on the 14th floor of a high-rise building. If I travel down 5 floors, then up 8 floors and then down 15 floors. Where am I?

**Answers**
**A1.** 25 + 20 = 45° C
**A2.** 8850 + 2550 = 11 400 metres
**A3.** +14 – 5 + 8 – 15 = 2nd floor

The image above shows hot water instantly evaporating in mid-air at –40 degrees. Why? At this chilly temperature, the air has zero percent
humidity and water will evaporate instantly. However, it is even chillier in space. **Absolute zero**, the temperature at which there is thought to be no
movement of any kind in the universe, is **0° Kelvin** or **–273° Centigrade**. The very chilly average temperature of the universe
due to cosmic radiation is 3° Kelvin or –270° Centigrade. If today's temperature where you live is a very comfortable
20° Centigrade, what would this be in degrees Kelvin?

Write these as number sentences and then calculate the answer.
**Q1.** From the 5th floor of a building, I go down in the elevator 3 floors, up 2 floors and then down 4 floors. Where am I?
**Q2.** I have $20. I owe my 2 friends $5 each. I then receive $15 from work. How much do I have?

**Answers**
**A1.** 5 – 3 + 2 – 4 = 0 (Ground floor)
**A2.** 20 – 5 – 5 + 15 = 25 ($25)

Trick candles, those novelty ones that re-light after you blow them out, contain a small amount of magnesium within the wick. When blown out, the candle's paraffin wax is still hot enough to re-ignite the magnesium which in turn re-lights the candle wick.