"Submarines can range in length from the size of a car to twice the length of a football field! Subs can submerge and surface -- go down and come up -- by controlling their own buoyancy."

"Buoyancy is all about something's ability to float. And things float or don't float depending on their density."

"Density is simply a way of comparing the mass or weight of something with its volume or size. To float, an object has to be less dense than the stuff it's floating in."

"So, subs can sometimes float and other times sink because they can change their density. There are two ways to change something's density: change its size or change its weight. In order to change its density, a submarine has to be able to add and subtract weight without changing its size. The way a submarine changes density is by taking in and spitting out sea water."

"Submarines have special tanks called ballast tanks that are filled with air when the sub is floating. When it needs to submerge, the sub fills these tanks with water. That makes it more dense and it sinks. When it's time to come back up, compressed air blows the water out of the ballast tanks. The sub becomes less dense and it rises to the surface."

"Now let's make a submarine in the comfort and privacy of our very own bathtub."

• Plastic soda bottle
• Balloon
• 3 feet of rubber tubing or some flexi-straws
• Tape
• Rubber bands
• Scissors
• Nail
• Coins
• A Grown up!!

1. Have your grown up use the scissors to make six holes across the top of the bottle and six holes across the bottom.
2. Now have your grown up use a nail to punch a hole in the bottle cap large enough for the tubing to fit through. (If you can't find rubber tubing, make your own by taping together some flexi-straws.)
3. Next, take your balloon and attach it with rubber bands to the end of the rubber tubing.
4. Put the balloon in the bottle and put the other end of the tubing through the bottle cap.
5. Screw on the cap.
6. Now, put some rubber bands around the bottle and slide some coins underneath.
7. Get in the tub!
8. Blow into the tubing and inflate the balloon, holding your finger over the end of the tube to keep it shut. Move the coins around so your sub floats nice and level.
9. To make your sub submerge, let the air out of the balloon. To make it rise back up to the surface, blow air into the balloon.

   BONUS FACTS!!
   (From our science consultant, Al Guenther)

   When considering why things float or sink, people tend to think only about weight; they forget about the equally important factor of volume. Density = weight divided by volume. Changing either weight or volume will change density. Increasing weight increases density, but decreasing volume also increases density.

   Unlike most real submarines, our model changes density by changing volume. People are confused by the fact that water enters the holes in the bottle, and they assume that this makes the model heavier.

   But, remember, a submerged object is supported by the water surrounding it (but not enough support to float it). Since the holes in the bottle remain open all the time, the water that enters the holes remains "connected" to the water outside the holes and continues to support the model. In most real subs the holes that let the water enter can be closed to seal the water inside the sub.

   So, the weight of our model really consists only of the bottle, the balloon, and the tube (the weight of the air is negligible). But you increase and decrease the volume of the model by making the balloon larger and smaller.

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