Surface tension

I don't know why I have never dealt with surface tension here: it is in at least two of my books. Here is how I have dealt with it in What On Earth?

Water has a property called surface tension. The why of surface tension is hard top explain, so the simple explanation is that surface tension is caused by polar water molecules all pulling each other together. Don’t worry too much about the why: the main thing is that the effects are amazing.

For example, water forms drops that stick together. Without surface tension, there would be no raindrops. Next, lots of insects and spiders can walk across a water surface. In fact, you can even make a paper clip sit on a water surface, but it isn’t floating, not as we usually mean ‘float’!

Many of the strange things that water does depend the effects of surface tension. This is a complicated idea, but you can see surface tension in the way wet hair clings together or the bristles of a paint brush stick to each other, but better, you can demonstrate it this way. 

You need two paper clips and a glass of water. Follow the pictures that you see here: bend one paper clip into an L shape. Use this clip to gently lay another paperclip on the top of the water. The surface of the water bends under the weight of the paperclip like stretched rubber, but doesn’t let it through.

You need to pick up one paper clip with the other. The next part needs a steady hand, or it won't work. You are going to lower the unbent paper clip onto the water surface like this:

The next step is to push the supporting paper clip down. With any sort of luck, the supported paper clip will 'float'. Let me say again, this is not really floating.

If the trick doesn’t work, pull the paperclip out, dry it carefully and rub a tiny bit of grease on the paperclip before trying again.

If you look closely at the reflections coming off the water in the photo on the right, you can see how the water surface is bent. To push the water surface out of shape you must use force.

If the paperclip can’t exert enough force, it can’t stretch the surface enough to let the paperclip slip through. All the same, one drop of detergent in the water, and all the magic goes! (No, I won’t explain why!)