system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why are wooden barrels "barrel-shaped" (i.e., wider across the middle than at the top and bottom) rather than cylindrical? Wouldn't cylindrical containers be easier to build, store, and transport?
gpt
Barrels that are full are VERY heavy. That's a lot of liquid.  So you want things that are easier to handle, not just move. And barrels have that bulge that makes they way better for this than a cylinder that's shaped like the cardboard in a toilet paper roll.

First, a barrel that has a rounded-outward middle is actually much easier to roll due to one point hitting the ground rather than the whole width of the cylinder. There's a lot less friction and a lot more control to do stuff like turn direction easily and quickly, something that's harder to do with a true cylinder shape.

And then it's a lot easier to push upright when it's lying down. You can rock it to start it tilting up, and then push a little more once it's rocking... and up it goes. A cylinder is a LOT tougher - you have to pretty much lift it instead.

But wait! There's more! They're easier to build really strongly too!

Barrels are made by steaming arranged pieces of slightly-angle-cut wood under pressure, bending them into a curve at both ends, and then jamming a couple iron hoops over their outsides from top and bottom, squeezing both towards the middle until that bulge is really squeezed tight. The curve in the wood provides the extra outward force, pressing hard against that iron hoop, to keep the whole thing together and snug. 

Well, that hoop's a lot easier than something adjustable that could fit over a perfect cylinder and then be tightened. You don't get that barrel-shape's magical ability to create more tension by just sliding the hoops on both ends toward the centre to create more pressure - you have to somehow tighten the iron hoop you're putting on instead on a cylinder.... and that would be a lot harder to do without slipping. You'd need something like a ratchet or a knotted and welded cable, instead of a simple and easy-to-make hoop. So guess which is cheaper?

Easier to move a lot of liquid, easier to stand it up after moving it, and easier and cheaper to construct. Lots to love about ye old-timer barrels compared those sucky cylinders!