Scientists try to answer questions, for example:

Why are planets spherical?

What happens to heartbeat during exercise? 

Which is the best material for a football?

There are many different ways of answering these questions or enquiries. 

Scientists carry out fair tests. They consider all the things which they could change, for example: the material used to make a football, but keep all the other things the same, for example: the size of the football, the material it is filled with etc...

These are called FACTORS or VARIABLES. Only ONE factor or variable can be changed at a time, to keep a test FAIR.

Got that?  Let's have a look at fair tests, then.