Consider three fundamental "ingredients".

Sugar

Sugar is needed, specifically the kind of sugar that yeast can break down into alcohol, i.e. simple sugars such as Maltose and Fructose.

Yeast

Obviously we need yeast, to supply alcohol from fermenting sugars.

Flavour

Finally, we need flavour, which in the simplest beer is provided by more complex sugars and acids in the brew.

Sugar and most of the flavours in beer are provided by... [next slide]