This shows that the surface tension of the water is “strong” enough to hold up the paper clip.
Get the students to squat down to get eye level with the water. Observe what is happening. Is the paper clip a little bit down in the water? Or does the paper clip totally rest on top? Can they see kind of a surface or skin-like appearance on the very top of the water?
Ask the students what they think will happen if a drop of detergent is added to the Petri dish. Now add a drop of detergent to the Petri dish away from where the paper clip is. The paper clip will instantly sink.