1. What is a litre?

Think of a litre as the space occupied by a cube of side 10 cm.

2. How can I convert units from one system to another e.g. metres into feet?

There are so many different systems in different countries and cultures that it is impossilbe to remember them all. A useful website is called MegaConvertor.

3. Who started the metric system?

The French introduced the metric system shortly after the French Revolution in the 1790s.

4. How far is a metre?

A metre was initially defined a one ten millionth of the distance from the equator to the North Pole on a line running through Paris.

5. Which are the most commonly used prefixes?

Kilo − 1000 times the base unit
centi- 1/100th of the basic unit
milli- 1/1000th of the base unit

6. How heavy is a gram?

A gram is not much at all! 
A small drop of liquid would weigh just a few grams.

7. What is a tonne?

A tonne is a mass of 1000 kg, which is the weight of a small truck.

8. What is the difference between mass and weight?

For mathematical purposes the mass and the weight are the same.