Example:

"Boil thou first i' the charmed pot.
Double, double toil and trouble;
Fire burn and cauldron bubble.
Fillet of a fenny snake,
In the cauldron boil and bake."
(Act 4, Scene 1, Lines 9-13).


Explanation:

The cauldron does not actually boil, the ingredients inside of it is what boils. The words "double, double" mimic the sound of boiling ingredients.