Hi guys, I'm growing a chestnut tree at home and it's currently sitting inside on the window sill. It's doing quite well, but last night I noticed that a fairly big mushroom had actually appeared very close to the base of the trunk. What does it mean? What type of mushroom is it and why is it there?
I'm attaching two pictures, the tree and the mushroom
thanks

Garden soil contains enough of the root mass of mushrooms that on occasion fruiting bodies form.

Why is this tree living indoors? It belongs outside only. Or even better planted to field.

OK cool, do you know what mushroom that is?
The tree is inside because I planted it (from seed) last winter. I didn't expect it to survive but it did and it's doing quite well. One day when it's a big bigger I will plant it outside, maybe next year.

By the time your chestnut has spent a winter indoors (with out its dormancy), it will be dead.