Agatha Christie. Hollow (№ 227565)

... For the absent, blank stare she was hiding her true nature - she was not as stupid as everyone thought. Often she was just pretending not to understand and, doing some work, deliberately hesitated and smiled to herself when someone's impatient fingers snatched the work from her hands.
The sense of superiority warmed and delighted. Quite often it even amused by the secret consciousness that she was not as stupid as everyone thinks. She quite managed to herself, but preferred to hide it.