What you need:
Plastic soft drink bottle (1 litre size works great) and lid, sauce packet ( such as tomato sauce)
from a restaurant , water
What you do:
First you need to test your sauce packet to see if it floats or sinks. Fill a bowl with water and toss in your packet. If it floats or barely floats, great ! You can use this packet. If it sinks, well that just means that you get another trip to a restaurant to find the right packet.
Once you found a sauce packet that doesn't sink, scrunch the packet in half lengthwise and carefully push it into the soft drink bottle. Fill the bottle full to the brim with water and screw on the cap. Squeeze the sides of the bottle (you won't have to squeeze real hard) and hold the squeeze to make the packet sink. Let go and it rises.
Be sure to add pizzazz to your presentation by willing and commanding your packet to sink . "Sauce packet I command you to sink, you will sink! Now rise, rise, rise" Well I'm sure you get the idea. Have fun.
How it works
The packet floats because an air bubble gets trapped inside the packet when it's sealed. If the packet sinks when you test-float it, then the air bubble is too small to make it float.
As you squeeze the bottle and push the water against the floating packet, you compress the air bubble within the packet into a smaller space. This can happen because gases are more squishable than liquids. So as you make the bubble of air smaller, you are actually decreasing it's volume. By doing this you increase the density of the packet and it will sink. When you release the pressure on the bottle, the compressed air expands inside the packet and it will float back to the top of the bottle.