Leaf Gall??

My daughter found this outside. My son poked at it forever thinking it was some kind of curled up caterpillar, but when I googled fuzzy thing on leaf I got a leaf gall, whatever that is. I wonder if it is some kind of insect egg sack though. Why in the world would this just grow on a leaf??? Why is it fuzzy? The world may never know I guess. The kids have it on a shelf so they can "watch it".