To show the class that it is difficult to describe something:

Person A has a picture.

Person B stands at the whiteboard and tries to draw it based
on person A's description.  Note that person A is NOT allowed to
see what person B is doing.

Possibly persons B, C and D.

Point out that a successful strategy involves:
  * starting with the broad outline;
  * dividing the picture into segments;
  * using clear language, avoiding ambiguities;
  * including ways to check for correctness.