They observe some humans summoning a magical spirit one day. 

It's hard to tell all the details when you have to watch in shifts over the course of several weeks and when you can't hear anything they're saying, but the overall picture seems clear. First, their barn burned down. Then, they stood outside watching it sadly for a while. Then a woman went away into the forest, and wrote some things on the ground, lots and lots of detailed human words. Then a magical spirit appeared. Then the magical spirit walked back with her and replaced the barn and filled up the new barn with food. Then she had sex with the magical spirit. Then the magical spirit disappeared.

It makes sense, in the way human things often don't make sense. It seems like the kind of thing you could perhaps try yourself.

They carefully copy the human words onto the ground in a fairy circle. They carefully draw a circle in the dirt around the words.