What are varibles?

A variable is something that can be changed.

In Math.

x+10= 15

x is the variable, which can be replaced by 5.

In an experiment:

How do maggots appear on a piece of meat?

You will need a controlled.
And a Variable.

Controlled: Kept in a Sealed Jar.
Variable: will be kept on the open, just on a table.

Then to compare it, you will need another variable, there are 2 variables.

Controlled: sealed Jar
Variable: Inside a Jar with holes on it.

You will find that the Sealed jar does not contain any maggots because the flies cannot get to the meat.

And the variables will prove that since the flies had some type of access to it, there will be maggots.