A Charlotte Mason Moment
Take away his slate and let him read history.
“The brain, or some portion of the brain, becomes exhausted when any given function has been exercised too long. The child has been doing sums for some time, and is getting unaccountably stupid: take away his slate and let him read history, and you find his wits fresh again. Imagination, which has had no part in sums, is called into play by the history lesson, and the child brings a lively unexhausted power to his new work. School time-tables are usually drawn up with a view to give the brain of the child variety of work; but the secret of weariness in children often show in the home schoolroom is, that no such judicious change of lessons is contrived.” (Home Education by Charlotte M. Mason Vol. 1, p. 24)