I use a faucet and pail analogy with a full pail representing proper exposure.
Say it takes one minute to fill a pail with water. If you open the faucet (Aperature) to double the flow and then you cut that time (shutter speed) in half to thirty seconds, You'll have exactly the proper amount water. You don't even have to look. Conversely, if you halve the flow it will take twice the time, two minutes to get the same amount.
This analogy can be extended futher in either direction and the consequences of abandoning the rules of reciprocity are obvious, either not enough water (underexposure) or wet shoes (overexposure).
The real pisser is trying to explain how to determine "proper" exposure.