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.