Part of the show Coral Reefs and Creatures of the Deep Sea
What you Need
It also works with a ketchup packet weighted with a paperclip instead of the pen lid
What to do
1- If you are using the penlid, use some plasticine to block the hole in the top, then add plasticine to the bottom of the lid until it just floats
2- Fill the bottle up to the top
3- Float the lid or sachet in the bottle.
4- Put the lid on the bottle
What may happen
When you squeeze the bottle you should find that the diver ( the penlid / sachet ) sinks and then when you stop squeezing it should float again. You should be able to repeat this as often as you like.
Why does it happen?
If something is surrounded by a fluid, it gets an upthrust (push upwards) that is as strong as the weight of the liquid it is pushing out of the way. This means that if an object takes up more space than it's weight in water (is less dense than water) it will float, and if an object is smaller than it's weight in water it will sink.
Both the pen lid and the sachet have an air pocket in them - the one in the sachet is better hidden, but you may be able to see it if you hold the sachet up to the light.. This makes them quite a lot bigger and so gives them lots of upthrust, whilst only increasing their weight by a minute amount, so it makes them less dense.
If you squash the bottle, because the water is virtually incompressible this pressure is transferred to the air pocket, squashing it and making it smaller. This makes the penlid smaller so it is more dense and sinks.
If you stop squashing the air will expand again reducing the lid's density so it floats again.
How is this related to a real submarine?
Real submarines can affect whether they float or sink in a similar way. They have large tanks on their sides which they can fill with water to increase the submarine's density and sink, or fill with compressed air from tanks inside the submarine to reduce their density agian so they float again.