Mixing drinks can create some interesting flavors, but it can also cause some interesting science! 

We are going to mix two staples when it comes to drinks that are not water: cola and milk. Some people actually like that combination, but it is a combo that can become more of a science project than a beverage.

For this, all we need is a bottle of cola and some milk.  This works best if you take a little cola out of the bottle.  

Now, we need to add milk slowly. It is better to do this slowly because if you dump the milk quickly, it can foam over.  I used 1/3 cup of milk for this experiment.

Now, shut the bottle, give it a gentle swirl, and set it down.

This is where the magic happens! 

After a little bit, you will notice that the top of the bottle starts to become clearer, while a bunch of brown stuff falls to the bottom. If you let this continue happening, you will have a clear, yellowish fluid left over with a bunch of gunk accumulated at the bottom of the bottle. 

This no longer looks appetizing.  

So, what is happening in the bottle?

When you mix milk into the cola, the primary proteins in the milk, called caseins, coagulate or curdle and start to drop, thanks to the phosphoric acid in the pop.  That is called "precipitating" out of the mixture.  

This protein carries a positive charge.

Do you remember the old saying, "opposites attract?" That is the second part of what is going on here.

The positive charge of the casein protein attracts the brown, caramel color in the cola, causing the artificial color to precipitate to the bottom of the bottle with it.  

This leaves behind that yellowish, clear fluid with the milk proteins and cola colorings at the bottom in a sort of muddy-looking mess. This is because the things that precipitated out of the mixture are denser than the liquid left behind.

This is still safe to drink, but it won't taste as good as a fresh pop, and it certainly does not look as good as a normal pop.