You will learn about the principles of buoyancy and especially neutral buoyancy in this task. Divers, submersibles and submarines alter the air levels in their buoyancy systems to change their density. In this challenge, you will need to create a neutrally buoyant pen lid submarine!

Activity steps

1. Remove any labels from your bottle so that you can see what is happening.
2. Fill the bottle to the very top with water. It’s probably best to do this activity with the bottle standing in your kitchen sink, as there are likely to be some small spills.
3. Fix a small piece of modeling clay to the end of the point on the pen lid. Start with quite a small piece as you can more easily add some more. Retrieving the pen lid from the bottom of the bottle is slightly trickier.
4. Put the pen lid into the bottle, modeling clay end first. It should just barely float, i.e. about 10cm from the top of the bottle.
5. If the lid floats too much, ask an adult to squeeze the sides of the bottle gently, until there is enough of the pen top poking out to grab a hold of. You can then add a little more modelling clay to help it sink a little.
6. If the pen lid sinks to the bottom, you will need to retrieve the pen lid from the bottom of the bottle and take off a little modeling clay. The best way of retrieving the pen lid is for an adult to place a thumb over the opening, turn the bottle upside down, and then remove their thumb for just long enough for the pen lid to come out.
7. Now screw on the bottle cap nice and tight. Remember to top up the water bottle if you have lost any water getting the amount of modeling clay right.
8. Now to see if you can make your pen lid submarine rise and fall. Squeeze the sides of the bottle hard. This should make the lid sink. If you stop squeezing, the lid will rise again.
9. Your challenge is to apply just the right amount of pressure to make the pen lid float in the middle of the bottle.

More ideas

Can you experiment with different factors that might affect how the pen lid floats and sinks? Does the size of the bottle make any difference or how full it is with water? Why might this be the case?

• Clear 1 to 1.5 liter plastic soda bottle
• Pen lid (without a hole in the top)
• Some modeling clay or poster putty such as Blu-Tack

This activity is all about density. When you squeeze the sides of the bottle, the air trapped inside the pen lid is compressed. This makes the pen lid more dense than the water, as the air bubble is smaller. As it is now less dense than the water, the pen lid sinks.

When you release the sides of the bottle, the air bubble is no longer compressed and the volume of the air increases. This makes the pen lid less dense than the water, so it rises.