This powerful technique, used for helping students to develop their understanding of a particular concept, involves slowly revealing pairs of statements, ideas, pictures (whatever) one of which embodies a particular concept and one of which doesn't. Make the fact that you are trying to get over a particular concept explicit, and challenge students to articulate the concept.
It may well be that they formulate an understanding of the concept without knowing the name for the concept. This is what makes the technique potentially very powerful - they can "understand" something before having a name for it.
You can then lead a discussion about how each of the earlier guesses relate to the "correct" concept, strengthening understanding of that concept.
Head up the columns of the pairs as "Example of this concept" and "Not an example of this concept" on the whiteboard, then slowly reveal the pairs one by one, with thinking time in between. For younger learners you might use "What's the big idea?" with Yes and No columns instead.