If a drink that's being carbonated is placed in a sealed container, the space between the liquid and the cap fills up with carbon dioxide. As the concentration of carbon dioxide increases, the gas starts to get pushed into solution and the bubbles stay in the water. This makes the drink carbonated. If the cap was loose, the gas would escape and you would get "flat" pop. As long as the yeast have enough sugar to eat, they keep making carbon dioxide. When the yeast run out of sugar, they start another metabolic process called fermentation. This process produces alcohol as a waste product. This is why yeast are used to produce the alcohol found in beer. We refrigerate our ginger beer so that the yeast don't multiply so much that they run out of food, and thus produce alcohol instead of carbon dioxide.

• pot filled with 2 L of water
• 1.5 tablespoon of powdered ginger or 0.5 lbs fresh ginger root
• 1/4 lime
• 1 1/4 cups of sugar
• 1.5 tablespoons cream of tartar
• 0.5 tablespoons baker's yeast
• cheese cloth
• funnel and mixing spoon
• 2 L jug with cap or cork

1. Boil the water and add the ginger, juice squeezed from the lime, sugar, and cream of tartar.
2. Let the mixture cool until it is lukewarm.
3. Add baker's yeast and mix well.
4. Cover pot and let stand for 6 hours.
5. To bottle the ginger beer:
   • strain the liquid through the cheesecloth or coffee filter by setting it on top of the funnel and slowly pouring the liquid into the bottle. (it's OK if some chunks slip through, they will settle to the bottom).
   • be sure to leave an air space at the top of the bottle to collect the gas so it won't explode.
6. Cap the jug tightly and put it into the refrigerator (this is to slow the activity of the yeast down; if you didn't you might produce alcohol which would give the ginger beer an unpleasant taste).
7. Wait two days. When you take the ginger beer out, look for the bubbles.
8. Slowly unscrew the top of the bottle and listen for the POP sound as the gas escapes.
9. You can flavour your ginger beer in many different ways by adding fresh, chopped mint leaves when you put the ginger in or using honey to replace the sugar.

Yeasts are a type of fungus. They are unicellular organisms, meaning that the whole organism is made up of only one cell. These organisms are not at all complex, and they have no systems. Multicellular organisms, on the other hand, are much more complex. Often they have groups of cells that work together to perform a specific task. For example, your organs are just large groups of cells that form tissues, and those tissues together form an organ. Of course, when scientists are studying things like metabolism, it is much easier to study it in an organism as simple as a unicellular organism. Yeasts are very often used for studies in the lab because of their simplicity.

The world is full of unicellular organisms. A few examples are amoeba, paramecia, bacteria, etc.