Floating Paper Clip

The results of the "Ship Shape" experiment might be confusing to you. Clay is denser than water, and sinks when it's shaped into a ball. But if you make a boat shape, it floats! In this experiment, you'll see that even things that are the same shape sometimes float, and sometimes don't.

Drop a paper clip into a glass of water. The clip sinks. But now bend the end of the clip upwards so that you can use it as a handle. Carefully place the clip on the surface of the water, so that it floats (you may have to try a few times). Take a careful look at the water around the floating paper clip. Here's the puzzle: the density of the metal in the paper clip didn't change, and neither did the density of the water. So why does the clip sink in the first case and float in the second?

The answer is that the very top (surface) of water is different. The surface molecules crowd together, creating a "skin" that is actually denser than the rest of the water. This effect is called "surface tension". If you look at the water around the floating paper clip, you'll see that it doesn't look flat - the paper clip has caused the skin to stretch. Surface tension isn't a very strong effect, so it only helps lightweight things to float, like water bugs, leaves, or pieces of paper.