Hay is basically grass clippings bundled together to feed to grass-eating animals when there isn't any.
Great care must be taken to collect hay when it is as dry as possible and store it in such a way as to dry it further and keep it dry because wet hay rots. Decay produces heat, and because hay is also a great insulator that heat can even reach the ignition point for the dry parts of the hay.
It tends to smolder, since it is predicated on being fairly wet at first and the middle where it is hottest will have bad airflow until the whole "being on fire" part dries the rest out, but it will set your barn on fire easily enough, and it's a real devil to put out, since you're usually trying to spray water through dozens of feet of hay to where the core of the problem actually is.
Do you ever sit and think sometimes how weird it is, to know all these things and have all these skills from your earlier life that have, like, no relevance or use in your current life?
It's weird to me how much I know about pigs, tractors and corn/soybeans/wheat that just doesn't come up much when you're sitting at a computer typing all day. (Hay bales should be stacked "sideways", to allow air to flow up through them, so that they will dry safely and not spontaneously combust. With straw it is fine to stack it any which way; some people prefer to stack it with the wires up so that it is easier to walk on, others the same as hay for consistency.)