My younger son is 7 years old. I was recently watching him when he was just doodling on a white board. He drew a square, and wrote a number "5" on it. Then he drew squiggly lines coming out of the bottom of the square. He said they were roots. He dubbed the drawing "the square root of 5".
My 13-year-old son has several loose baby teeth. In some cases, the adult teeth have already come in next to them. He started yesterday to work on one that was really bothering him. After a lot of grunting and groaning, he finally got it out. This emboldened him to go to work on the next one, and the next one. He pulled three of his own teeth in one day.