Beer is alcoholic drink made from fermented grain. It has four basic ingredients:
Water – comes from the sky, lives in rivers, lakes and taps.
Malt – grain (typically barley) that has started to sprout and then been put in a kiln to kill it and provide the sugar.
Hops – a green flower that provides the bitterness, flavour and aroma to the beer (also acts as a preservative).
Yeast – a fungi that eats sugar and as a waste product creates alcohol (and carbon dioxide).
Other stuff can be in added to beer but this is all you need to make magic happen.