Lexi: "Our earth rotates on its own axis in the course of one day and the sun only shines on one half of the earth at once. Where the sun shines it is daytime, whereas on the other side it is night. If it was the same time everywhere, it would be daylight at midday in some countries and in others pitch black.

In the 19th Century the world was divided up into different time zones - as a result it is light at midday and dark at midnight in all countries. So, when you fly on holiday you sometimes have to change your watch so that it shows the time that prevails in that particular country."

Bernie: "Exactly Lexi. The difference to the next time zone is always one hour. For example, Switzerland, Austria and Germany all have the same time. But when you fly to Portugal on holiday, you have to turn your watch back an hour."

Lexi: "Bernie is right: If you travel to West the clock gets turned back and if you travel to the East it is turned forward. For example, when people are eating lunch in Thailand, we are just getting up here in Switzerland. When Bernie and I fly to Canada, we pass through several time zones. Incidentally, some countries are so big that they have many time zones - for instance the USA or Russia (with no less than 11 time zones!).

Do you know what is cool? You can imagine you are travelling through time when you are flying. Next time ask you parents whether you have to turn your watch back or forward. Then you know whether you are travelling into the past or into the future!"