The napkin flight

The paper napkin burns from top to bottom. When the paper napkin is almost burnt off, the remaining, burnt, light paper tissue lifts off abruptly and rises quickly.

A napkin consists of several layers. However, you only take one layer and cut out a quarter of it. Now roll it up into a tube 2 to 3 cm in diameter. Place this tube vertically on a dry plate. Now light the top edge with a lighter or match. Make sure that the tube does not fall over when you light it, otherwise the burnt napkin will collapse. As not all napkins are suitable for this experiment, you should carry out a preliminary test to check whether the napkin is suitable for this experiment and whether it can be carried out in class. For large napkins, you may have to shorten the tube slightly.

• a paper napkin,
• a plate,
• a lighter or matches,

To increase the stability of the tube, you can support the tube with longitudinal bends. This experiment is quite uncomplicated and very impressive. However, caution is required when handling fire, so the teacher should demonstrate the experiment themselves.

What is technically behind this experiment

The light ash is carried upwards by the rising warm air. The rising of the warm air is the result of the lower density of the warm air in the vicinity of the tube compared to the colder air outside. The chimney effect probably also plays a role in the experiment: the rising warm air inside the tube has to be replaced by colder air flowing in from below. This air is also heated and flows upwards and so on. When the unburned residue cools down, it slowly sinks again.

What the experiment has to do with everyday life

The rising of warm air can be recognized in everyday life, for example, by the grey traces on the wall above radiators. The principle of rising warm air is better known from the rotation of a Christmas pyramid. The hot air balloon rises according to the same principle. Another everyday reference is the question of how wind is created. When air is heated by the sun, it expands, becomes less dense and rises. Cooler air takes its place. In this way, air starts to move and wind is created - with the rotation of the earth creating additional turbulence.

Nicole Schäfer & Sabrina Stark