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Herding Water
After watching the marvels of how the dogs herd the
sheep in New Zealand, I thought you might like to do a little herding of your
own. This does not require sheep or dogs, but it is still fun. I know that
this experiment has been around for a LONG time since many of you have
recommended that I use it, but hopefully there are some of you out there that it
will be new for.
You will need:
Lay a sheet of wax paper on a flat surface. The size
of the sheet is not critical, but about a foot square should be plenty. Dip
your finger into some water and shake it at the paper to get one or more drops
of water to fall. Do this several times, until you have at least 10 or 12 drops
on the paper. Dip your toothpick into the water to get it wet and you are ready
to begin.
The idea is to get all of the small drops joined
together into one large drop, but you are not allowed to move the wax paper. No
fair picking up the edges of the paper and letting gravity to the work.
Instead, you will use the toothpick to lead the drops. What? How can you lead
a drop of water? Let's try it and see. Be sure your toothpick is wet. Bring
the tip of the toothpick to just touch the side of one of the drops of water.
Notice how the water flows toward it. Now slowly and gently move the toothpick
towards one of the other water drops. The drop you touched will follow along
with the toothpick. When it touches the new drop, they will flow together into
one. Then you can lead this drop to the next one. As the drop gets bigger, you
will have to be more careful to keep it from breaking into two pieces. You
might want to see who can join the most drops into one.
Why does this happen? Well, water is very sticky
stuff. Dip your finger into the water and you will see that some of it sticks
to your skin, making your finger wet. Dip a piece of paper into the water and
some will stick to the paper, making the paper wet. Water will even stick to
itself. That is what causes surface tension on water. With all of the water
molecules pulling on each other, they pull the water into rounded drops. When
you touch the water drop with the toothpick, the water sticks to the toothpick
and to the water that is already sticking to the toothpick. When you pull the
toothpick, the water slides across the wax paper as it is pulled along.
Why doesn't the water stick to the wax paper? Wax,
like oil and grease, is a substance that the water does not stick to. If you
tried this trick on a sheet of notebook paper, it would not work very well.
Instead, the water would stick to the paper too, leaving a wet trail behind the
drop as you move the toothpick. Well, I had better finish this and send it out
so I can get a little sleep and be ready for shows tomorrow.
Have a good week.
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