The teacher of the TOE (theoretical foundations of electrical engineering) asked us: "Why does an alternating current pass through a condenser and a constant one does not?" and drew a drawing on the board: a horizontal straight, divided in the middle by two vertical lines (condensator).
Here is the alternating current. - painted a sinusoid with the axis of a horizontal line passing in the maximum amplitude of the condenser and going further.
It is a constant current. - drawn a horizontal straight, parallel to the main and leaning in the condenser. The current is constant and cannot pass through the condenser.
P.S At the third grade, everyone knew it was a joke.