As a result, I have discovered that there is such a thing as an algebra headache.
It starts with a feeling that the brain is being overworked. Then concepts start to get a little foggy. The forehead starts to feel kind of full. Then there is a mild feeling of panic that seems to ascend from somewhere around the base of the skull. I know it's an emotion but it seems to accompany this physical sensation. Then you are distracted from the topic the grey matter is working on because the entire head starts to feel pressured.
Then... the actual ache starts. It's time for caffeine and motrin, and maybe even a little break. A funny or lighthearted activity is recommended.