Pocket whirlpool and liquid hourglass

Capture the power of nature in a bottle!

30 minutes
Pocket whirlpool and liquid hourglass


  • Carefully review the general safety advice in the instruction book before starting the experiment.
  • Avoid contact with food and dishware. Thoroughly wash or dispose of any dishware you use.

Step-by-step instructions

  • Tornado in a bottle

Take two empty plastic bottles. You can empty the bottles from the box set or find your own. Fill one of them with water. You can also add a metallic coloring to the water or experiment with other liquids. Now, join the bottles with the double-sided bottle cap from the box. Try to pour the water from one bottle to the other. Not happening? Try swirling the bottles to create a watery “tornado.” Have fun!

  • Liquid hourglass

Would you like to make a liquid “sand clock”? Use modeling clay to fix two straws in the double-sided bottle cap, pointing in opposite directions. Now, fill the bottles with two liquids that have different densities and don’t mix (for instance, water and oil) and join them with the double-sided cap. If the bottle on top has a denser liquid (water), it will start dropping into the bottle on the bottom, and the liquid from the bottom bottle (oil) will start moving up. Did it work?


  • Dispose of solid waste together with household garbage.
  • Pour liquids down the sink. Wash with an excess of water.

Scientific description

  • Tornado in a bottle

When you invert a bottle full of water, you expect it to start leaking out. However, the movements of any body of water depend on the pressure that surrounding materials put on it. The air pressure outside the bottle resists the flow of water, so the pressure from above must be greater than the outside air pressure for the water to begin to flow. If the bottleneck is large enough, the water will leak out until the pressure inside the bottle becomes lower than the air pressure outside the bottle. At that moment, an air bubble will interrupt the flow, enter the bottle, rise to the top, and increase the pressure so the flow continues.

In this case, when the bottle is simply turned upside down, nothing happens because the bottleneck is too small: the surface tension keeps the water inside the bottle and blocks the air from entering. In the liquid vortex you create, however, the pressure in the center is lower than it is near the edges. The air from the lower bottle overcomes the water pressure and surface tension, forming a steady air tunnel to the top of the water-filled bottle. As a result, the water receives additional air pressure from above and falls into the lower bottle.

  • Liquid hourglass

In a large, open vessel, a less dense liquid will rise above a denser liquid if they do not mix. This is the result of buoyancy, which is a combination of gravitational force and the fact that pressure in liquids acts in all directions. But with two bottles and only a small hole between them, surface tension, the force which acts to minimize the area of contact between two liquids, opposes the forming of the drops of water which drowns in the oil and the drops of oil which rise above the water. Instead of rapidly streaming past each other, the liquids ever so slowly seep into each other until they finally switch places altogether.