Here is my completely-unscientific-due-to-my-small-sample size-observation for the day. I am now working on teaching my 7th and 8th children how to read and I realize that there is a phenomenon that happens every single time I do this. That is, when a young child is working very hard, such as when sounding out words, at some point into the process, every single day, they start to yawn. It's as if the brain is working so hard that it is requiring extra oxygen to process what it is being asked to do.
The other thing I have noticed is that once the yawning starts to happen, I know I can only get just a little bit more concentrated effort out of the child, if I push very far past that moment, then it becomes counter-productive. Either the child gets upset or they start to make mistakes or they get silly. It just isn't worth it... believe me, I've tried.
Thus I can't help myself from engaging in a little pro-homeschooling propaganda. You see, the length of active engagement and participation is different from child to child and even from day to day for the same child. I just never know where a child's brain will be on any given day. Some days, a child is really on top of things; the words come easily and we cover quite a bit. Other days, well, that same child can read three words and be maxed out. A child's brain just doesn't seem to learn in a straight line; it's more of a back and forth, jump and stall process. Since I am teaching a child to read one-on-one, I can adjust expectations accordingly. It's a beautiful thing.