25th May 2001 at 01:00

pound;4.75 for pack of four Commotion, Unit 11, Tannery Road, Tonbridge, Kent TN9 1RF. Tel: 01732 773399.

A pack of Windbags contains four large, inflatable, polythene "sausages". Tie up one end, inflate them and, the manufacturers suggest, they can be used to learn about the properties of air, and to build giant balloon animals or shapes. At more than 2m in length and 44 litres in volume, Windbags appear to be more of a toy than a teaching aid.

Instructions show several activities which involve using exhaled air to inflate the Windbag. For such a large volume this is likely to cause dizziness and possibly fainting. There are warnings about the dangers of suffocation with small children but nothing about the effects of hyperventilation or words of caution to asthma sufferers. Additionally, disinfection should be carried out if the Windbag is to be used by more than one person, and safety matters need to be considered before using it in the classroom.

In one experiment, a challenge is set to see how many breaths it takes to inflate the bag. This could be more than 20 although instructions suggest t can be achieved with a single breath: tricky, if not amazing.

In another experiment, the experimenter holds the bag a small distance from his or her mouth and blows hard into the open end which, with practice, fills the bag in one go. The explanation is that it demonstrates Bernoulli's principle whereby the column of moving air blown into the Windbag creates an area of low pressure that draws in surrounding air to inflate the bag. Whether an inquisitive pupil will be convinced is another matter, and there are other much more believable ways of demonstrating the phenomenon.

Windbags are certainly unusual and can be used to demonstrate the airflow around the school buildings or the effects of air pressure, although both effects can equally be achieved with balloons. Older pupils could use the Windbags to collect large volumes of exhaled air in spirometry experiments if specialised Douglas bags are not available. Inventive teachers will find other uses, but their main application could be as decorations for the Christmas disco.

Cliff Porter Cliff Porter is a freelance writer on science education

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