1. How heavy is a gram?

A gram is not much at all! 
A teaspoon of medicine would weigh about 5 grams.

2. Who started the metric system?

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

3. 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.

4. 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

5. What is a litre?

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

6. What is a tonne?

A tonne is a mass of 1000 kg

7. What is the difference between mass and weight?

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