Eiffel tower

The Eiffel tower in Paris has including the tv antenna an height of 321 meter. It is made out of 7 million kg steel. Somebody would like to build a scale model of this tower with a weight of 1 kg. How tall will this scale model be?

Rocket launch

Why are the European Ariane rockets launched from the Korou site in French-Guyana, South-America, and not simply from some place in Europe where the rockets are partly built?

Thunder and lightning

Why does a flash of lightning take only an instant, while the corresponding thunder can last for a few seconds? You might have expected that that the sound is caused by the flash, and that the thunder should therefore be audible also for an instant.