How to Make a Ketchup Packet Cartesian Diver

Diving Ketchup Magic Trick (Cartesian Diver)

A small pile of ketchup packets

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Place a ketchup packet in a bottle of water and make it rise and fall at your command, as if by magic. Of course, the magic involves some basic science, and squeezing the bottle. With a little practice, you can make it appear that you aren't squeezing the bottle. If you are performing this demonstration as a magic trick you can say you are using the power of your mind to move the ketchup. Pretend to be concentrating very hard, using your incredible powers of telekinesis.

How the Trick Works

An air bubble is trapped inside a ketchup packet when it is sealed at the factory. If the bubble is big enough, it makes the packet float in water. When you squeeze the bottle, the water won't compress but the air bubble inside the ketchup packet is squeezed and becomes smaller. The size of the packet is reduced, but its mass remains unchanged. Density is mass per volume, so squeezing the bottle increases the density of the ketchup packet. When the density of the packet is more than the density of the water the packet will sink. When you release the pressure on the bottle the air bubble expands and the ketchup packet rises. Here's how to do the diving ketchup trick and how it works.

What You Need

  • Plastic bottle
  • Ketchup packet from a fast food restaurant
  • Water


  1. Assemble your materials.
  2. Drop the ketchup packet into the bottle.
  3. Seal the lid of the bottle.
  4. Fill the bottle with water. The ketchup packet should float somewhere in the middle of the bottle.
  5. If it sinks, try using a different packet (the size of the air bubble inside the ketchup packet varies slightly) or else dump out the water and ketchup, bend the packet slightly so that it can hold an air bubble when you fill the bottle again, and refill the bottle. Trap an air bubble on a heavy packet and tap the bottle until you knock off enough bubbles to get the packet to the middle of the bottle.
  6. Squeeze the bottle to cause the ketchup packet to sink.
  7. Relax your grip on the bottle to cause the packet to float.