As you dropped the water onto the penny, what shape did the water make?

Water has a REALLY cool property called cohesion which basically means it likes to stick to itself.  We'll learn WHY it likes to stick to itself in the next section, but right now all you need to know is that water likes water.  If a water droplet can connect with another water droplet, it will. 

It's a lot like playing Red Rover with your friends.  You hold on tight to each other's hands and when someone comes crashing in, you try not to let them through.  If you hold strong enough, then that person gets to join your group and make it bigger.  But if the person runs too hard and fast then it breaks a part your line. 

That's what water does.  If you're careful enough with your drops then the water will connect itself into a larger sphere rather than spilling over.  But if the drops come in too hard or too fast (or eventually if there's just too many of them) then the water dome will spill over.

But why does water like to stick to itself and what are other examples of cohesion?  Let's find out!

Paperclips will usually sink to water.  But if you are REALLY CAREFUL you can get them to float on top. 

Using what we learned from Challenge 1, why do you think the paperclip can float?  

Have you ever belly flopped in water?  How did it feel?
  
We already learned that water likes to stick to itself. That's called cohesion.  Cohesion of water works especially well on the surfaces of water.  We call this surface tension.  Water molecules will pull on each other and stick together as tight as they can, acting like a stretched skin on the top of the water. 

When you use a fork to very carefully lay the paperclip on the water, the surface tension of the water helps keep it up. 

This is how water skippers and other small bugs walk on water!  

If you were to take water droplet and split it in half, then split it in half again, then split it again and again and keep splitting it until you got down the smallest drop that could still be considered water, what would it look like?  

Well, eventually you would get a single water molecule.  It would look a lot like this picture. Water is made out of three atoms (basically three small balls):  one large Oxygen atom and two large Hydrogen atoms.  In chemistry, we say that the chemical composition of water is H2O (two Hydrogen and one Oxygen atoms).

That in and of itself is important, but not super interesting. BUT!  What happens when two Hydrogen atoms join an Oxygen atom?  Things start to become a little more fascinating...

How do things get more fascinating?  

Well... It turns out that Oxygen atoms are a little bit greedy.   Hydrogen atoms and Oxygen atoms stay together because they share electrons.  It's kind of like if you had a bag of chips and your friend had salsa, you would both want to stay near each other because you each have something the other wants.  If you share, you're both happy!  That's how Hydrogen and Oxygen feel - when they share their electrons, they are both happier!

But... Oxygen is bigger.  And it's greedy.   It likes keeping the electrons and only shares occasionally.  (You can imagine a friend that keeps the chips and salsa in front of them and only lets you reach in to get some every once in a while).  

This means that the Oxygen atom has more electrons while the Hydrogen atoms have less.  Electrons have a negative charge, so the Oxygen atom has a slight negative charge while the Hydrogen atoms have a slight positive charge. 
We call this a "polar molecule" - meaning it has a positive and a negative end.

What happens when you have two things next to each other, each with a positive and negative end?  Think of water molecules like little mini magnets.  The water molecules flip around and stick so the positive end of one molecule touces the negative end of the other.  This is why water likes to stick to itself!

Try Challenges One and Two again but with soapy water. 

Does it work better or worse? 

- Could you get your paperclip to float?
- Did your penny hold more or fewer drops?

Have you ever tried to mix water and oil?  What happens?  

Water is a polar molecule- meaning that each molecule has a positive end and a negative end, so it likes to stick to itself.  Oil is a nonpolar molecule - meaning that it doesn't have a positive or negative end so it doesn't like to stick to water.  If you mix oil and water, they separate and won't mix.

Soap is interesting, because it has a polar head (loves water) and a nonpolar tail (hates water and likes oils/fats).  Because of this, one side of a soap molecule will stick to the water, but the other side won't stick to another water molecule. As the soap gets between the water molecules, it weakens the cohesion of water and breaks the bonds between the water molecules.

When you get oil or grease on your hands or stains in your clothes, water isn't enough to get it clean.  Water doesn't mix with oils and fats or grease.  But soap likes both water and oils!  So when you wash your hands with soap or use soap in your laundry, the water can now attach itself to the oils and stains to remove it and get everything clean!

Also... this is where soap becomes even more magical.  Many germs are wrapped in a fatty, oily coat.  When you wash your hands with soap, the oil loving tails of the soap molecules break apart the germ's fatty coat.  This breaks apart the germs so they can't stay together anymore and they get washed down the drain.

Get two paper cups and a piece of yarn. (You'll want a way to attach things).

Can you make a bridge between the two cups using the string then transfer the water from one cup to the other?

Why does the water follow the string instead of flowing straight down?

Have you ever tried pouring water out of a cup or a bowl and have it run down the side instead? 

What about a time when you were washing dishes or standing in the shower and water runs down your arm?

In addition to wanting to stick to itself, water also likes sticking to other things.  We call this adhesion.   

The fact that water likes to stick to other things is what allows things to become wet and damp.  When you got your string wet, then you could carefully pour the water out of the cup and the water would stick to itself and stick to the string to travel down to the other cup.

Get two paper cups and your bendable straw.  Set one of the cups up higher than the other.

Without moving the cups, can you figure out a way to get the water to go from one cup to the other by creating a siphon?

The water wants to go down hill, but it may need to go uphill before it can go down.   How did you get the siphon started?

Siphons are really interesting bits of science and engineering.  In order to get a siphon started, the straw or tube or pipe must be full of water.  Gravity will pull the water in the longer "leg" of the straw down and as it does so, the water in the shorter "leg" of the straw will go up and over the bend to keep a continuous stream of water going.