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Virtual Science Fair

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Cartesian Divers

Have science fun as a family! Complete activities with parental supervision.

  • Empty plastic bottle (we used 1 liter), with labels removed
  • Water
  • Ketchup packet (must float in water)
  1. First, test your ketchup packet. Place it in a bowl or cup of water. Does it float? Not all packets will float in water, but we need one that does for this activity.
  2. Over a sink or a basin, fill the bottle to the top with water until it overflows (we want all of the air out of the bottle).
  3. Carefully push the ketchup packet into the bottle – be careful not to squeeze it open! If too much water came out of the bottle, top it off with water again.
  4. Tightly screw on the top.
  5. Squeeze the sides of the bottle. What happens to the ketchup packet?
  6. Be sure to take a picture or video to share in the Facebook comments on the Buffalo Museum of Science or Tifft Nature Preserve pages!

What’s it all about?

This experiment is all about two science concepts- buoyancy and density. Buoyancy is an objects’ tendency to float in a liquid. In many ketchup packets, there is
a little bubble inside that causes it to float. Density is a comparison between an object’s mass and its volume. Basically, it measures how much stuff is smushed into
a given area.


Remember that tiny bubble? If you squeeze the bottle hard enough, you also squeeze the ketchup packet and the bubble inside gets smaller. As the bubble is squeezed and shrinks, the packet sinks because it becomes more dense and less buoyant!

Try It!
  • How can we change up this experiment?
  • Do different condiments have the same effect?
  • Does the size of the bottle change anything?
  • What other variables can you think of to change?
  • If you have an eye dropper, try that instead of a ketchup packet.
    What do you see as you squeeze the bottle?

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