A tun is a special word used to talk about how much beer can fit in a really big barrel. It's like when you fill up a big bucket with water and you need to know how much water is in it, but instead of water, it's beer! A tun is a really big barrel that is used to hold a lot of beer.
But how do they know how much beer can fit in a tun? Well, a tun is actually a specific size that has been used for a really long time, so people know exactly how much it can hold. In fact, a tun is so big that it can hold about 252 gallons of beer! That's more beer than you could ever drink by yourself.
Nowadays, we don't use tuns as much as we used to because we have other ways of measuring how much beer is in a container. But, tuns are still important for people who make beer and want to know how much they are making. So, remember that a tun is just a big barrel that holds a lot of beer!