Next introduce an episode that builds from the idea of delays to contributions in and out of step, and so to the idea of superposition.
By altering the delay between a source and its imitator, you can make them out of step, by altering the trip time from source to imitator (set by speed at which the path is covered and the length of the path).
The imitator could be at a detector. If there is more than one path from source to detector, then contributions from those paths might end up in or out of step, as a result of the trip times for those paths. Add the contributions to find the resultant displacement at the detector. The amplitude at the detector could be large, zero, or anything in between. It all depends on whether the contributions are in or out of step.