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 that 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.