A while back, Eli and Liza excitedly talked about water striders. “They walk on water, dad!”
As a scientist, I never miss a teachable opportunity. “You know why water striders can walk on water?” I asked them. “It’s because of surface tension. I’ll show you!”
How to Demonstrate Surface Tension
This easy and fun experiment will be sure to captivate absolutely no one.
- Glass of water
- A piece of toilet paper
- One or more kids who will be unimpressed by your experiment
- Tear off an amount of toilet paper that’s small enough to fit in the glass but large enough for the needle to rest on it.
- After you make sure your fingers and the needle are dry, place the needle on the toilet paper.
- When the kids ask why you’ve taken toilet paper out of the bathroom, explain that you’re going to use it to make a needle float.
- Place the toilet paper and needle on top of the water.
- Tell the kids, “Watch! Watch what happens when the toilet paper sinks.”
- Wait for the toilet paper to sink.
- Keep waiting.
- When the unimpressed kids wander off, tell them, “Don’t go anywhere! It’s about to happen!”
- Poke at the toilet paper to try to make it sink.
- Poke some more, until the needle sinks and you realize that you’ve splashed water on the needle, ruining the experiment.
- Say to the kids, “No, it’ll work, I swear. Just let me try again.” Ignore them rolling their eyes.
- Dry off the needle. Repeat steps 1 through 4.
- This time, be more careful when you poke the toilet paper and make it sink.
- With the needle floating on the water, excitedly say, “Kids! Come look! The water’s surface tension is holding it up.”
- Start to explain that the liquid is made up of molecules that are like small magnets and pull towards each other, which lets them hold up the needle. Stop when you realize the kids aren’t listening.
- Instead say, “If you look carefully at the water you can see where the top of the water is being pushed down by how the lights reflect off of it.”
- When you move the glass to better catch the light, slosh the water so that the needle sinks.
- Give up and let the kids wander away.