Tornado in a Bottle

You need:

• Two big plastic soft drink bottles with caps
• hot glue
• file
• drill
• food color (optional)

Build Instructions:

• Empty the bottles :-) Remove the paper wrap
  
• Remove the caps and drill a hole in each of them (about 10mm diameter)
  
• Roughen up the top side of the caps (around the holes) with a file
• Now it should look like this:
  
  

• Glue the 2 caps together (see notes at the end).
  
  

• Now fill one of the bottle with water (about half full) and add food color if you like
• Put the double cap on one of your bottles
• Turn the other bottle upside down and screw it into the other end of your double cap

Create the Tornado:

• Turn the 2 bottles upside down. Only little water will flow through the hole in the caps.
• Now swirl the water around in the upper bottle
• When the tornado builds up, hold the bottle still again.
• Watch. Enjoy. Repeat.
  
  

Some Notes:

• Gluing plastic is tricky. Most glues won't work. If you do not have a special plastic glue, hot glue is the next best alternative I think. Some tips:
• Make sure the gluing area is roughened up.
  
• Make sure your hot glue gun is properly hot.
• Put on the hot glue quickly on one cap and the press the other cap onto the first one firmly.
• Add some extra glue around the gap the where the two caps meet
  
• When screwing onto the bottles, make sure the glued spot is not twisted.
• When creating the tornado, try not to bend it. Swirl the whole thing.

UPDATE:

To get around the gluing problems, I came up with the following:

• Use hot glue as described above
• When the glue is hard, fix the caps using 2 or 3 small screws around the hole
• Make sure the screw are not too close to outer edge of the caps, it will not close properly and leak then

Your caps should look like this then: