The grains used in beer are usually malted barley (barley grains that have been soaked in water). The starches from these are turned into sugars which interact with the yeast to make alcohol during the fermentation process.
Hops are the flowers of the hop plant, Humulus lupulus. The job of hops is to work as a bittering, flavouring and stability agent within the beer. They’re usually added to the boil stage in brewing, where the necessary compounds within them are brought out under heat.
Yeast is the third really important ingredient. Yeast is a fungal microorganism – a living being that’s used in the fermentation stage – converting carbohydrates (sugars) into alcohol.
Water is our final ingredient. Not too surprising, really – without it, beer wouldn’t be much of a drink. Beer is around 90% water, which is why it’s so refreshing on a hot day (and why pubs have bathrooms, of course). There’s not much else to say about water in beer, other than it obviously needs to be really high-quality. Top brewers will concern themselves with the sourcing of their water, because the mineral content within will affect the final taste. Running it straight from the tap probably won’t make for the world’s best beer.