1. Fill one of the bottles with water.

     2. Turn upside down one of the bottles and join both bottles mouths with an adhesive tape. (you should put enough tape in order to prevent the loss of water)

     3. Turn upside down both bottles and wheel them a little bitte. Here is the tornado! Enjoy!

What you have just seen during this experiment is a simulated tornado. When you wheel the bottles, it's created a vortex and due to the velocity gradient of water, an upside down cone appears. This happens because the water that is more close to the bottle hole has higher flow speed.
In the particular case of the real tornados, these are formed due to the encounter of cold air and hot humidified air. Due to its lower density, hot air ascends, forming and vortex and, finally, a whirlwind. Inside the tornado, velocity can reach 400 km/hr. Try it and simulate a tornado at home!